Merge branch 'jl/read-tree-m-dry-run'

* jl/read-tree-m-dry-run:
  Teach read-tree the -n|--dry-run option
  unpack-trees: add the dry_run flag to unpack_trees_options

14 files changed, 202 insertions, 143 deletions

diff --git a/Documentation/git-read-tree.txt b/Documentation/git-read-tree.txt
index 26fdadc642..46a96f2313 100644
--- a/Documentation/git-read-tree.txt
+++ b/Documentation/git-read-tree.txt
@@ -53,6 +53,11 @@ OPTIONS
 	trees that are not directly related to the current
 	working tree status into a temporary index file.
 
+-n::
+--dry-run::
+	Check if the command would error out, without updating the index
+	nor the files in the working tree for real.
+
 -v::
 	Show the progress of checking files out.
 
diff --git a/builtin/read-tree.c b/builtin/read-tree.c
index 93c92814cf..df6c4c8819 100644
--- a/builtin/read-tree.c
+++ b/builtin/read-tree.c
@@ -130,6 +130,7 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 		  PARSE_OPT_NONEG, exclude_per_directory_cb },
 		OPT_SET_INT('i', NULL, &opts.index_only,
 			    "don't check the working tree after merging", 1),
+		OPT__DRY_RUN(&opts.dry_run, "don't update the index or the work tree"),
 		OPT_SET_INT(0, "no-sparse-checkout", &opts.skip_sparse_checkout,
 			    "skip applying sparse checkout filter", 1),
 		OPT_SET_INT(0, "debug-unpack", &opts.debug_unpack,
@@ -219,7 +220,7 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 	if (unpack_trees(nr_trees, t, &opts))
 		return 128;
 
-	if (opts.debug_unpack)
+	if (opts.debug_unpack || opts.dry_run)
 		return 0; /* do not write the index out */
 
 	/*
diff --git a/t/lib-read-tree.sh b/t/lib-read-tree.sh
new file mode 100644
index 0000000000..abc2c6f57f
--- /dev/null
+++ b/t/lib-read-tree.sh
@@ -0,0 +1,43 @@
+#!/bin/sh
+#
+# Helper functions to check if read-tree would succeed/fail as expected with
+# and without the dry-run option. They also test that the dry-run does not
+# write the index and that together with -u it doesn't touch the work tree.
+#
+read_tree_must_succeed () {
+    git ls-files -s >pre-dry-run &&
+    git read-tree -n "$@" &&
+    git ls-files -s >post-dry-run &&
+    test_cmp pre-dry-run post-dry-run &&
+    git read-tree "$@"
+}
+
+read_tree_must_fail () {
+    git ls-files -s >pre-dry-run &&
+    test_must_fail git read-tree -n "$@" &&
+    git ls-files -s >post-dry-run &&
+    test_cmp pre-dry-run post-dry-run &&
+    test_must_fail git read-tree "$@"
+}
+
+read_tree_u_must_succeed () {
+    git ls-files -s >pre-dry-run &&
+    git diff-files -p >pre-dry-run-wt &&
+    git read-tree -n "$@" &&
+    git ls-files -s >post-dry-run &&
+    git diff-files -p >post-dry-run-wt &&
+    test_cmp pre-dry-run post-dry-run &&
+    test_cmp pre-dry-run-wt post-dry-run-wt &&
+    git read-tree "$@"
+}
+
+read_tree_u_must_fail () {
+    git ls-files -s >pre-dry-run &&
+    git diff-files -p >pre-dry-run-wt &&
+    test_must_fail git read-tree -n "$@" &&
+    git ls-files -s >post-dry-run &&
+    git diff-files -p >post-dry-run-wt &&
+    test_cmp pre-dry-run post-dry-run &&
+    test_cmp pre-dry-run-wt post-dry-run-wt &&
+    test_must_fail git read-tree "$@"
+}
diff --git a/t/t1000-read-tree-m-3way.sh b/t/t1000-read-tree-m-3way.sh
index ca8a4098fa..babcdd2343 100755
--- a/t/t1000-read-tree-m-3way.sh
+++ b/t/t1000-read-tree-m-3way.sh
@@ -72,6 +72,7 @@ In addition:
 
 '
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-read-tree.sh
 . "$TEST_DIRECTORY"/lib-read-tree-m-3way.sh
 
 ################################################################
@@ -137,7 +138,7 @@ test_expect_success \
     '3-way merge with git read-tree -m, empty cache' \
     "rm -fr [NDMALTS][NDMALTSF] Z &&
      rm .git/index &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 # This starts out with the first head, which is the normal
@@ -146,9 +147,9 @@ test_expect_success \
     '3-way merge with git read-tree -m, match H' \
     "rm -fr [NDMALTS][NDMALTSF] Z &&
      rm .git/index &&
-     git read-tree $tree_A &&
+     read_tree_must_succeed $tree_A &&
      git checkout-index -f -u -a &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 : <<\END_OF_CASE_TABLE
@@ -211,7 +212,7 @@ test_expect_success '1 - must not have an entry not in A.' "
      rm -f .git/index XX &&
      echo XX >XX &&
      git update-index --add XX &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -219,7 +220,7 @@ test_expect_success \
     "rm -f .git/index NA &&
      cp .orig-B/NA NA &&
      git update-index --add NA &&
-     git read-tree -m $tree_O $tree_A $tree_B"
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B"
 
 test_expect_success \
     '2 - matching B alone is OK in !O && !A && B case.' \
@@ -227,14 +228,14 @@ test_expect_success \
      cp .orig-B/NA NA &&
      git update-index --add NA &&
      echo extra >>NA &&
-     git read-tree -m $tree_O $tree_A $tree_B"
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B"
 
 test_expect_success \
     '3 - must match A in !O && A && !B case.' \
     "rm -f .git/index AN &&
      cp .orig-A/AN AN &&
      git update-index --add AN &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -243,7 +244,7 @@ test_expect_success \
      cp .orig-A/AN AN &&
      git update-index --add AN &&
      echo extra >>AN &&
-     git read-tree -m $tree_O $tree_A $tree_B"
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B"
 
 test_expect_success \
     '3 (fail) - must match A in !O && A && !B case.' "
@@ -251,7 +252,7 @@ test_expect_success \
      cp .orig-A/AN AN &&
      echo extra >>AN &&
      git update-index --add AN &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -259,7 +260,7 @@ test_expect_success \
     "rm -f .git/index AA &&
      cp .orig-A/AA AA &&
      git update-index --add AA &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -268,7 +269,7 @@ test_expect_success \
      cp .orig-A/AA AA &&
      git update-index --add AA &&
      echo extra >>AA &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -277,7 +278,7 @@ test_expect_success \
      cp .orig-A/AA AA &&
      echo extra >>AA &&
      git update-index --add AA &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -285,7 +286,7 @@ test_expect_success \
     "rm -f .git/index LL &&
      cp .orig-A/LL LL &&
      git update-index --add LL &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -294,7 +295,7 @@ test_expect_success \
      cp .orig-A/LL LL &&
      git update-index --add LL &&
      echo extra >>LL &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -303,7 +304,7 @@ test_expect_success \
      cp .orig-A/LL LL &&
      echo extra >>LL &&
      git update-index --add LL &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -311,7 +312,7 @@ test_expect_success \
      rm -f .git/index DD &&
      echo DD >DD &&
      git update-index --add DD &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -319,7 +320,7 @@ test_expect_success \
      rm -f .git/index DM &&
      cp .orig-B/DM DM &&
      git update-index --add DM &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -327,7 +328,7 @@ test_expect_success \
      rm -f .git/index DN &&
      cp .orig-B/DN DN &&
      git update-index --add DN &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -335,7 +336,7 @@ test_expect_success \
     "rm -f .git/index MD &&
      cp .orig-A/MD MD &&
      git update-index --add MD &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -344,7 +345,7 @@ test_expect_success \
      cp .orig-A/MD MD &&
      git update-index --add MD &&
      echo extra >>MD &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -353,7 +354,7 @@ test_expect_success \
      cp .orig-A/MD MD &&
      echo extra >>MD &&
      git update-index --add MD &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -361,7 +362,7 @@ test_expect_success \
     "rm -f .git/index ND &&
      cp .orig-A/ND ND &&
      git update-index --add ND &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -370,7 +371,7 @@ test_expect_success \
      cp .orig-A/ND ND &&
      git update-index --add ND &&
      echo extra >>ND &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -379,7 +380,7 @@ test_expect_success \
      cp .orig-A/ND ND &&
      echo extra >>ND &&
      git update-index --add ND &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -387,7 +388,7 @@ test_expect_success \
     "rm -f .git/index MM &&
      cp .orig-A/MM MM &&
      git update-index --add MM &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -396,7 +397,7 @@ test_expect_success \
      cp .orig-A/MM MM &&
      git update-index --add MM &&
      echo extra >>MM &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -405,7 +406,7 @@ test_expect_success \
      cp .orig-A/MM MM &&
      echo extra >>MM &&
      git update-index --add MM &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -413,7 +414,7 @@ test_expect_success \
     "rm -f .git/index SS &&
      cp .orig-A/SS SS &&
      git update-index --add SS &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -422,7 +423,7 @@ test_expect_success \
      cp .orig-A/SS SS &&
      git update-index --add SS &&
      echo extra >>SS &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -431,7 +432,7 @@ test_expect_success \
      cp .orig-A/SS SS &&
      echo extra >>SS &&
      git update-index --add SS &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -439,7 +440,7 @@ test_expect_success \
     "rm -f .git/index MN &&
      cp .orig-A/MN MN &&
      git update-index --add MN &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -448,7 +449,7 @@ test_expect_success \
      cp .orig-A/MN MN &&
      git update-index --add MN &&
      echo extra >>MN &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -456,7 +457,7 @@ test_expect_success \
     "rm -f .git/index NM &&
      cp .orig-A/NM NM &&
      git update-index --add NM &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -465,7 +466,7 @@ test_expect_success \
      cp .orig-B/NM NM &&
      git update-index --add NM &&
      echo extra >>NM &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -474,7 +475,7 @@ test_expect_success \
      cp .orig-A/NM NM &&
      git update-index --add NM &&
      echo extra >>NM &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -483,7 +484,7 @@ test_expect_success \
      cp .orig-A/NM NM &&
      echo extra >>NM &&
      git update-index --add NM &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 test_expect_success \
@@ -491,7 +492,7 @@ test_expect_success \
     "rm -f .git/index NN &&
      cp .orig-A/NN NN &&
      git update-index --add NN &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -500,7 +501,7 @@ test_expect_success \
      cp .orig-A/NN NN &&
      git update-index --add NN &&
      echo extra >>NN &&
-     git read-tree -m $tree_O $tree_A $tree_B &&
+     read_tree_must_succeed -m $tree_O $tree_A $tree_B &&
      check_result"
 
 test_expect_success \
@@ -509,7 +510,7 @@ test_expect_success \
      cp .orig-A/NN NN &&
      echo extra >>NN &&
      git update-index --add NN &&
-     test_must_fail git read-tree -m $tree_O $tree_A $tree_B
+     read_tree_must_fail -m $tree_O $tree_A $tree_B
 "
 
 # #16
@@ -522,7 +523,7 @@ test_expect_success \
     echo E16 >F16 &&
     git update-index F16 &&
     tree1=`git write-tree` &&
-    git read-tree -m $tree0 $tree1 $tree1 $tree0 &&
+    read_tree_must_succeed -m $tree0 $tree1 $tree1 $tree0 &&
     git ls-files --stage'
 
 test_done
diff --git a/t/t1001-read-tree-m-2way.sh b/t/t1001-read-tree-m-2way.sh
index 680d992f22..acaab07fac 100755
--- a/t/t1001-read-tree-m-2way.sh
+++ b/t/t1001-read-tree-m-2way.sh
@@ -21,6 +21,7 @@ In the test, these paths are used:
         yomin   - not in H nor M
 '
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-read-tree.sh
 
 read_tree_twoway () {
     git read-tree -m "$1" "$2" && git ls-files --stage
@@ -94,7 +95,7 @@ echo '+100644 X 0	yomin' >expected
 test_expect_success \
     '4 - carry forward local addition.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      git update-index --add yomin &&
      read_tree_twoway $treeH $treeM &&
@@ -106,7 +107,7 @@ test_expect_success \
 test_expect_success \
     '5 - carry forward local addition.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      echo yomin >yomin &&
      git update-index --add yomin &&
@@ -120,7 +121,7 @@ test_expect_success \
 test_expect_success \
     '6 - local addition already has the same.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      git update-index --add frotz &&
      read_tree_twoway $treeH $treeM &&
@@ -131,7 +132,7 @@ test_expect_success \
 test_expect_success \
     '7 - local addition already has the same.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      echo frotz >frotz &&
      git update-index --add frotz &&
@@ -144,7 +145,7 @@ test_expect_success \
 test_expect_success \
     '8 - conflicting addition.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      echo frotz frotz >frotz &&
      git update-index --add frotz &&
@@ -153,7 +154,7 @@ test_expect_success \
 test_expect_success \
     '9 - conflicting addition.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      echo frotz frotz >frotz &&
      git update-index --add frotz &&
@@ -163,7 +164,7 @@ test_expect_success \
 test_expect_success \
     '10 - path removed.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      echo rezrov >rezrov &&
      git update-index --add rezrov &&
@@ -174,7 +175,7 @@ test_expect_success \
 test_expect_success \
     '11 - dirty path removed.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      echo rezrov >rezrov &&
      git update-index --add rezrov &&
@@ -184,7 +185,7 @@ test_expect_success \
 test_expect_success \
     '12 - unmatching local changes being removed.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      echo rezrov rezrov >rezrov &&
      git update-index --add rezrov &&
@@ -193,7 +194,7 @@ test_expect_success \
 test_expect_success \
     '13 - unmatching local changes being removed.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      echo rezrov rezrov >rezrov &&
      git update-index --add rezrov &&
@@ -208,7 +209,7 @@ EOF
 test_expect_success \
     '14 - unchanged in two heads.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      echo nitfol nitfol >nitfol &&
      git update-index --add nitfol &&
@@ -221,7 +222,7 @@ test_expect_success \
 test_expect_success \
     '15 - unchanged in two heads.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      echo nitfol nitfol >nitfol &&
      git update-index --add nitfol &&
@@ -235,7 +236,7 @@ test_expect_success \
 test_expect_success \
     '16 - conflicting local change.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      echo bozbar bozbar >bozbar &&
      git update-index --add bozbar &&
@@ -244,7 +245,7 @@ test_expect_success \
 test_expect_success \
     '17 - conflicting local change.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      echo bozbar bozbar >bozbar &&
      git update-index --add bozbar &&
@@ -254,7 +255,7 @@ test_expect_success \
 test_expect_success \
     '18 - local change already having a good result.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      cat bozbar-new >bozbar &&
      git update-index --add bozbar &&
@@ -266,7 +267,7 @@ test_expect_success \
 test_expect_success \
     '19 - local change already having a good result, further modified.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      cat bozbar-new >bozbar &&
      git update-index --add bozbar &&
@@ -279,7 +280,7 @@ test_expect_success \
 test_expect_success \
     '20 - no local change, use new tree.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      cat bozbar-old >bozbar &&
      git update-index --add bozbar &&
@@ -291,7 +292,7 @@ test_expect_success \
 test_expect_success \
     '21 - no local change, dirty cache.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      cat bozbar-old >bozbar &&
      git update-index --add bozbar &&
@@ -302,7 +303,7 @@ test_expect_success \
 test_expect_success \
     '22 - local change cache updated.' \
     'rm -f .git/index &&
-     git read-tree $treeH &&
+     read_tree_must_succeed $treeH &&
      git checkout-index -u -f -q -a &&
      sed -e "s/such as/SUCH AS/" bozbar-old >bozbar &&
      git update-index --add bozbar &&
@@ -359,7 +360,7 @@ test_expect_success \
 
 test_expect_success \
     'a/b (untracked) vs a, plus c/d case test.' \
-    'test_must_fail git read-tree -u -m "$treeH" "$treeM" &&
+    'read_tree_u_must_fail -u -m "$treeH" "$treeM" &&
      git ls-files --stage &&
      test -f a/b'
 
@@ -386,7 +387,7 @@ test_expect_success \
 
 test_expect_success \
     'a/b vs a, plus c/d case test.' \
-    'git read-tree -u -m "$treeH" "$treeM" &&
+    'read_tree_u_must_succeed -u -m "$treeH" "$treeM" &&
      git ls-files --stage | tee >treeMcheck.out &&
      test_cmp treeM.out treeMcheck.out'
 
@@ -401,7 +402,7 @@ test_expect_success '-m references the correct modified tree' '
 	echo a >file-a &&
 	git add file-a &&
 	git ls-tree $(git write-tree) file-a >expect &&
-	git read-tree -m HEAD initial-mod &&
+	read_tree_must_succeed -m HEAD initial-mod &&
 	git ls-tree $(git write-tree) file-a >actual &&
 	test_cmp expect actual
 '
diff --git a/t/t1002-read-tree-m-u-2way.sh b/t/t1002-read-tree-m-u-2way.sh
index a4a17e0017..a847709a13 100755
--- a/t/t1002-read-tree-m-u-2way.sh
+++ b/t/t1002-read-tree-m-u-2way.sh
@@ -9,6 +9,7 @@ This is identical to t1001, but uses -u to update the work tree as well.
 
 '
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-read-tree.sh
 
 compare_change () {
 	sed >current \
@@ -56,8 +57,8 @@ test_expect_success \
 test_expect_success \
     '1, 2, 3 - no carry forward' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
-     git read-tree -m -u $treeH $treeM &&
+     read_tree_u_must_succeed --reset -u $treeH &&
+     read_tree_u_must_succeed -m -u $treeH $treeM &&
      git ls-files --stage >1-3.out &&
      cmp M.out 1-3.out &&
      sum bozbar frotz nitfol >actual3.sum &&
@@ -69,11 +70,11 @@ test_expect_success \
 test_expect_success \
     '4 - carry forward local addition.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo "+100644 X 0	yomin" >expected &&
      echo yomin >yomin &&
      git update-index --add yomin &&
-     git read-tree -m -u $treeH $treeM &&
+     read_tree_u_must_succeed -m -u $treeH $treeM &&
      git ls-files --stage >4.out || return 1
      git diff -U0 --no-index M.out 4.out >4diff.out
      compare_change 4diff.out expected &&
@@ -87,12 +88,12 @@ test_expect_success \
 test_expect_success \
     '5 - carry forward local addition.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
-     git read-tree -m -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
+     read_tree_u_must_succeed -m -u $treeH &&
      echo yomin >yomin &&
      git update-index --add yomin &&
      echo yomin yomin >yomin &&
-     git read-tree -m -u $treeH $treeM &&
+     read_tree_u_must_succeed -m -u $treeH $treeM &&
      git ls-files --stage >5.out || return 1
      git diff -U0 --no-index M.out 5.out >5diff.out
      compare_change 5diff.out expected &&
@@ -107,10 +108,10 @@ test_expect_success \
 test_expect_success \
     '6 - local addition already has the same.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo frotz >frotz &&
      git update-index --add frotz &&
-     git read-tree -m -u $treeH $treeM &&
+     read_tree_u_must_succeed -m -u $treeH $treeM &&
      git ls-files --stage >6.out &&
      test_cmp M.out 6.out &&
      check_cache_at frotz clean &&
@@ -123,11 +124,11 @@ test_expect_success \
 test_expect_success \
     '7 - local addition already has the same.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo frotz >frotz &&
      git update-index --add frotz &&
      echo frotz frotz >frotz &&
-     git read-tree -m -u $treeH $treeM &&
+     read_tree_u_must_succeed -m -u $treeH $treeM &&
      git ls-files --stage >7.out &&
      test_cmp M.out 7.out &&
      check_cache_at frotz dirty &&
@@ -141,27 +142,27 @@ test_expect_success \
 test_expect_success \
     '8 - conflicting addition.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo frotz frotz >frotz &&
      git update-index --add frotz &&
-     if git read-tree -m -u $treeH $treeM; then false; else :; fi'
+     if read_tree_u_must_succeed -m -u $treeH $treeM; then false; else :; fi'
 
 test_expect_success \
     '9 - conflicting addition.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo frotz frotz >frotz &&
      git update-index --add frotz &&
      echo frotz >frotz &&
-     if git read-tree -m -u $treeH $treeM; then false; else :; fi'
+     if read_tree_u_must_succeed -m -u $treeH $treeM; then false; else :; fi'
 
 test_expect_success \
     '10 - path removed.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo rezrov >rezrov &&
      git update-index --add rezrov &&
-     git read-tree -m -u $treeH $treeM &&
+     read_tree_u_must_succeed -m -u $treeH $treeM &&
      git ls-files --stage >10.out &&
      cmp M.out 10.out &&
      sum bozbar frotz nitfol >actual10.sum &&
@@ -170,28 +171,28 @@ test_expect_success \
 test_expect_success \
     '11 - dirty path removed.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo rezrov >rezrov &&
      git update-index --add rezrov &&
      echo rezrov rezrov >rezrov &&
-     if git read-tree -m -u $treeH $treeM; then false; else :; fi'
+     if read_tree_u_must_succeed -m -u $treeH $treeM; then false; else :; fi'
 
 test_expect_success \
     '12 - unmatching local changes being removed.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo rezrov rezrov >rezrov &&
      git update-index --add rezrov &&
-     if git read-tree -m -u $treeH $treeM; then false; else :; fi'
+     if read_tree_u_must_succeed -m -u $treeH $treeM; then false; else :; fi'
 
 test_expect_success \
     '13 - unmatching local changes being removed.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo rezrov rezrov >rezrov &&
      git update-index --add rezrov &&
      echo rezrov >rezrov &&
-     if git read-tree -m -u $treeH $treeM; then false; else :; fi'
+     if read_tree_u_must_succeed -m -u $treeH $treeM; then false; else :; fi'
 
 cat >expected <<EOF
 -100644 X 0	nitfol
@@ -201,10 +202,10 @@ EOF
 test_expect_success \
     '14 - unchanged in two heads.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo nitfol nitfol >nitfol &&
      git update-index --add nitfol &&
-     git read-tree -m -u $treeH $treeM &&
+     read_tree_u_must_succeed -m -u $treeH $treeM &&
      git ls-files --stage >14.out &&
      test_must_fail git diff -U0 --no-index M.out 14.out >14diff.out &&
      compare_change 14diff.out expected &&
@@ -221,11 +222,11 @@ test_expect_success \
 test_expect_success \
     '15 - unchanged in two heads.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo nitfol nitfol >nitfol &&
      git update-index --add nitfol &&
      echo nitfol nitfol nitfol >nitfol &&
-     git read-tree -m -u $treeH $treeM &&
+     read_tree_u_must_succeed -m -u $treeH $treeM &&
      git ls-files --stage >15.out &&
      test_must_fail git diff -U0 --no-index M.out 15.out >15diff.out &&
      compare_change 15diff.out expected &&
@@ -242,27 +243,27 @@ test_expect_success \
 test_expect_success \
     '16 - conflicting local change.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo bozbar bozbar >bozbar &&
      git update-index --add bozbar &&
-     if git read-tree -m -u $treeH $treeM; then false; else :; fi'
+     if read_tree_u_must_succeed -m -u $treeH $treeM; then false; else :; fi'
 
 test_expect_success \
     '17 - conflicting local change.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo bozbar bozbar >bozbar &&
      git update-index --add bozbar &&
      echo bozbar bozbar bozbar >bozbar &&
-     if git read-tree -m -u $treeH $treeM; then false; else :; fi'
+     if read_tree_u_must_succeed -m -u $treeH $treeM; then false; else :; fi'
 
 test_expect_success \
     '18 - local change already having a good result.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo gnusto >bozbar &&
      git update-index --add bozbar &&
-     git read-tree -m -u $treeH $treeM &&
+     read_tree_u_must_succeed -m -u $treeH $treeM &&
      git ls-files --stage >18.out &&
      test_cmp M.out 18.out &&
      check_cache_at bozbar clean &&
@@ -272,11 +273,11 @@ test_expect_success \
 test_expect_success \
     '19 - local change already having a good result, further modified.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo gnusto >bozbar &&
      git update-index --add bozbar &&
      echo gnusto gnusto >bozbar &&
-     git read-tree -m -u $treeH $treeM &&
+     read_tree_u_must_succeed -m -u $treeH $treeM &&
      git ls-files --stage >19.out &&
      test_cmp M.out 19.out &&
      check_cache_at bozbar dirty &&
@@ -292,10 +293,10 @@ test_expect_success \
 test_expect_success \
     '20 - no local change, use new tree.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo bozbar >bozbar &&
      git update-index --add bozbar &&
-     git read-tree -m -u $treeH $treeM &&
+     read_tree_u_must_succeed -m -u $treeH $treeM &&
      git ls-files --stage >20.out &&
      test_cmp M.out 20.out &&
      check_cache_at bozbar clean &&
@@ -305,11 +306,11 @@ test_expect_success \
 test_expect_success \
     '21 - no local change, dirty cache.' \
     'rm -f .git/index nitfol bozbar rezrov frotz &&
-     git read-tree --reset -u $treeH &&
+     read_tree_u_must_succeed --reset -u $treeH &&
      echo bozbar >bozbar &&
      git update-index --add bozbar &&
      echo gnusto gnusto >bozbar &&
-     if git read-tree -m -u $treeH $treeM; then false; else :; fi'
+     if read_tree_u_must_succeed -m -u $treeH $treeM; then false; else :; fi'
 
 # Also make sure we did not break DF vs DF/DF case.
 test_expect_success \
@@ -336,7 +337,7 @@ test_expect_success \
      rm -fr DF &&
      echo DF >DF &&
      git update-index --add DF &&
-     git read-tree -m -u $treeDF $treeDFDF &&
+     read_tree_u_must_succeed -m -u $treeDF $treeDFDF &&
      git ls-files --stage >DFDFcheck.out &&
      test_cmp DFDF.out DFDFcheck.out &&
      check_cache_at DF/DF clean'
diff --git a/t/t1004-read-tree-m-u-wf.sh b/t/t1004-read-tree-m-u-wf.sh
index eb8e3d4476..b3ae7d52c6 100755
--- a/t/t1004-read-tree-m-u-wf.sh
+++ b/t/t1004-read-tree-m-u-wf.sh
@@ -3,6 +3,7 @@
 test_description='read-tree -m -u checks working tree files'
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-read-tree.sh
 
 # two-tree test
 
@@ -29,7 +30,7 @@ test_expect_success 'two-way not clobbering' '
 
 	echo >file2 master creates untracked file2 &&
 	echo >subdir/file2 master creates untracked subdir/file2 &&
-	if err=`git read-tree -m -u master side 2>&1`
+	if err=`read_tree_u_must_succeed -m -u master side 2>&1`
 	then
 		echo should have complained
 		false
@@ -42,7 +43,7 @@ echo file2 >.gitignore
 
 test_expect_success 'two-way with incorrect --exclude-per-directory (1)' '
 
-	if err=`git read-tree -m --exclude-per-directory=.gitignore master side 2>&1`
+	if err=`read_tree_u_must_succeed -m --exclude-per-directory=.gitignore master side 2>&1`
 	then
 		echo should have complained
 		false
@@ -53,7 +54,7 @@ test_expect_success 'two-way with incorrect --exclude-per-directory (1)' '
 
 test_expect_success 'two-way with incorrect --exclude-per-directory (2)' '
 
-	if err=`git read-tree -m -u --exclude-per-directory=foo --exclude-per-directory=.gitignore master side 2>&1`
+	if err=`read_tree_u_must_succeed -m -u --exclude-per-directory=foo --exclude-per-directory=.gitignore master side 2>&1`
 	then
 		echo should have complained
 		false
@@ -64,7 +65,7 @@ test_expect_success 'two-way with incorrect --exclude-per-directory (2)' '
 
 test_expect_success 'two-way clobbering a ignored file' '
 
-	git read-tree -m -u --exclude-per-directory=.gitignore master side
+	read_tree_u_must_succeed -m -u --exclude-per-directory=.gitignore master side
 '
 
 rm -f .gitignore
@@ -84,7 +85,7 @@ test_expect_success 'three-way not complaining on an untracked path in both' '
 	echo >file2 file two is untracked on the master side &&
 	echo >subdir/file2 file two is untracked on the master side &&
 
-	git read-tree -m -u branch-point master side
+	read_tree_u_must_succeed -m -u branch-point master side
 '
 
 test_expect_success 'three-way not clobbering a working tree file' '
@@ -94,7 +95,7 @@ test_expect_success 'three-way not clobbering a working tree file' '
 	git checkout master &&
 	echo >file3 file three created in master, untracked &&
 	echo >subdir/file3 file three created in master, untracked &&
-	if err=`git read-tree -m -u branch-point master side 2>&1`
+	if err=`read_tree_u_must_succeed -m -u branch-point master side 2>&1`
 	then
 		echo should have complained
 		false
@@ -113,7 +114,7 @@ test_expect_success 'three-way not complaining on an untracked file' '
 	echo >file3 file three created in master, untracked &&
 	echo >subdir/file3 file three created in master, untracked &&
 
-	git read-tree -m -u --exclude-per-directory=.gitignore branch-point master side
+	read_tree_u_must_succeed -m -u --exclude-per-directory=.gitignore branch-point master side
 '
 
 test_expect_success '3-way not overwriting local changes (setup)' '
@@ -137,7 +138,7 @@ test_expect_success '3-way not overwriting local changes (our side)' '
 	git reset --hard &&
 
 	echo >>file1 "local changes" &&
-	git read-tree -m -u branch-point side-a side-b &&
+	read_tree_u_must_succeed -m -u branch-point side-a side-b &&
 	grep "new line to be kept" file1 &&
 	grep "local changes" file1
 
@@ -151,7 +152,7 @@ test_expect_success '3-way not overwriting local changes (their side)' '
 	git reset --hard &&
 
 	echo >>file2 "local changes" &&
-	test_must_fail git read-tree -m -u branch-point side-a side-b &&
+	read_tree_u_must_fail -m -u branch-point side-a side-b &&
 	! grep "new line to be kept" file2 &&
 	grep "local changes" file2
 
@@ -173,7 +174,7 @@ test_expect_success SYMLINKS 'funny symlink in work tree' '
 	git add a/b &&
 	git commit -m "we add a/b" &&
 
-	git read-tree -m -u sym-a sym-a sym-b
+	read_tree_u_must_succeed -m -u sym-a sym-a sym-b
 
 '
 
@@ -209,7 +210,7 @@ test_expect_success 'D/F setup' '
 test_expect_success 'D/F' '
 
 	git checkout side-b &&
-	git read-tree -m -u branch-point side-b side-a &&
+	read_tree_u_must_succeed -m -u branch-point side-b side-a &&
 	git ls-files -u >actual &&
 	(
 		a=$(git rev-parse branch-point:subdir/file2)
diff --git a/t/t1005-read-tree-reset.sh b/t/t1005-read-tree-reset.sh
index 849911683a..f53de79e56 100755
--- a/t/t1005-read-tree-reset.sh
+++ b/t/t1005-read-tree-reset.sh
@@ -3,6 +3,7 @@
 test_description='read-tree -u --reset'
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-read-tree.sh
 
 # two-tree test
 
@@ -22,13 +23,13 @@ test_expect_success 'setup' '
 '
 
 test_expect_success 'reset should work' '
-  git read-tree -u --reset HEAD^ &&
+  read_tree_u_must_succeed -u --reset HEAD^ &&
   git ls-files >actual &&
   test_cmp expect actual
 '
 
 test_expect_success 'reset should remove remnants from a failed merge' '
-  git read-tree --reset -u HEAD &&
+  read_tree_u_must_succeed --reset -u HEAD &&
   git ls-files -s >expect &&
   sha1=$(git rev-parse :new) &&
   (
@@ -37,13 +38,13 @@ test_expect_success 'reset should remove remnants from a failed merge' '
   ) | git update-index --index-info &&
   >old &&
   git ls-files -s &&
-  git read-tree --reset -u HEAD &&
+  read_tree_u_must_succeed --reset -u HEAD &&
   git ls-files -s >actual &&
   ! test -f old
 '
 
 test_expect_success 'Porcelain reset should remove remnants too' '
-  git read-tree --reset -u HEAD &&
+  read_tree_u_must_succeed --reset -u HEAD &&
   git ls-files -s >expect &&
   sha1=$(git rev-parse :new) &&
   (
@@ -58,7 +59,7 @@ test_expect_success 'Porcelain reset should remove remnants too' '
 '
 
 test_expect_success 'Porcelain checkout -f should remove remnants too' '
-  git read-tree --reset -u HEAD &&
+  read_tree_u_must_succeed --reset -u HEAD &&
   git ls-files -s >expect &&
   sha1=$(git rev-parse :new) &&
   (
@@ -73,7 +74,7 @@ test_expect_success 'Porcelain checkout -f should remove remnants too' '
 '
 
 test_expect_success 'Porcelain checkout -f HEAD should remove remnants too' '
-  git read-tree --reset -u HEAD &&
+  read_tree_u_must_succeed --reset -u HEAD &&
   git ls-files -s >expect &&
   sha1=$(git rev-parse :new) &&
   (
diff --git a/t/t1008-read-tree-overlay.sh b/t/t1008-read-tree-overlay.sh
index f9e00285db..4c50ed955e 100755
--- a/t/t1008-read-tree-overlay.sh
+++ b/t/t1008-read-tree-overlay.sh
@@ -3,6 +3,7 @@
 test_description='test multi-tree read-tree without merging'
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-read-tree.sh
 
 test_expect_success setup '
 	echo one >a &&
@@ -21,7 +22,7 @@ test_expect_success setup '
 '
 
 test_expect_success 'multi-read' '
-	git read-tree initial master side &&
+	read_tree_must_succeed initial master side &&
 	(echo a; echo b/c) >expect &&
 	git ls-files >actual &&
 	test_cmp expect actual
diff --git a/t/t1011-read-tree-sparse-checkout.sh b/t/t1011-read-tree-sparse-checkout.sh
index 20a50eba5b..018c3546b6 100755
--- a/t/t1011-read-tree-sparse-checkout.sh
+++ b/t/t1011-read-tree-sparse-checkout.sh
@@ -12,6 +12,7 @@ test_description='sparse checkout tests
 '
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-read-tree.sh
 
 test_expect_success 'setup' '
 	cat >expected <<-\EOF &&
@@ -43,7 +44,7 @@ test_expect_success 'setup' '
 '
 
 test_expect_success 'read-tree without .git/info/sparse-checkout' '
-	git read-tree -m -u HEAD &&
+	read_tree_u_must_succeed -m -u HEAD &&
 	git ls-files --stage >result &&
 	test_cmp expected result &&
 	git ls-files -t >result &&
@@ -52,7 +53,7 @@ test_expect_success 'read-tree without .git/info/sparse-checkout' '
 
 test_expect_success 'read-tree with .git/info/sparse-checkout but disabled' '
 	echo >.git/info/sparse-checkout &&
-	git read-tree -m -u HEAD &&
+	read_tree_u_must_succeed -m -u HEAD &&
 	git ls-files -t >result &&
 	test_cmp expected.swt result &&
 	test -f init.t &&
@@ -62,7 +63,7 @@ test_expect_success 'read-tree with .git/info/sparse-checkout but disabled' '
 test_expect_success 'read-tree --no-sparse-checkout with empty .git/info/sparse-checkout and enabled' '
 	git config core.sparsecheckout true &&
 	echo >.git/info/sparse-checkout &&
-	git read-tree --no-sparse-checkout -m -u HEAD &&
+	read_tree_u_must_succeed --no-sparse-checkout -m -u HEAD &&
 	git ls-files -t >result &&
 	test_cmp expected.swt result &&
 	test -f init.t &&
@@ -72,7 +73,7 @@ test_expect_success 'read-tree --no-sparse-checkout with empty .git/info/sparse-
 test_expect_success 'read-tree with empty .git/info/sparse-checkout' '
 	git config core.sparsecheckout true &&
 	echo >.git/info/sparse-checkout &&
-	test_must_fail git read-tree -m -u HEAD &&
+	read_tree_u_must_fail -m -u HEAD &&
 	git ls-files --stage >result &&
 	test_cmp expected result &&
 	git ls-files -t >result &&
@@ -90,7 +91,7 @@ test_expect_success 'match directories with trailing slash' '
 	EOF
 
 	echo sub/ > .git/info/sparse-checkout &&
-	git read-tree -m -u HEAD &&
+	read_tree_u_must_succeed -m -u HEAD &&
 	git ls-files -t > result &&
 	test_cmp expected.swt-noinit result &&
 	test ! -f init.t &&
@@ -99,7 +100,7 @@ test_expect_success 'match directories with trailing slash' '
 
 test_expect_success 'match directories without trailing slash' '
 	echo sub >.git/info/sparse-checkout &&
-	git read-tree -m -u HEAD &&
+	read_tree_u_must_succeed -m -u HEAD &&
 	git ls-files -t >result &&
 	test_cmp expected.swt-noinit result &&
 	test ! -f init.t &&
@@ -149,7 +150,7 @@ EOF
 
 test_expect_success 'match directory pattern' '
 	echo "s?b" >.git/info/sparse-checkout &&
-	git read-tree -m -u HEAD &&
+	read_tree_u_must_succeed -m -u HEAD &&
 	git ls-files -t >result &&
 	test_cmp expected.swt-noinit result &&
 	test ! -f init.t &&
@@ -165,7 +166,7 @@ test_expect_success 'checkout area changes' '
 	EOF
 
 	echo init.t >.git/info/sparse-checkout &&
-	git read-tree -m -u HEAD &&
+	read_tree_u_must_succeed -m -u HEAD &&
 	git ls-files -t >result &&
 	test_cmp expected.swt-nosub result &&
 	test -f init.t &&
@@ -175,7 +176,7 @@ test_expect_success 'checkout area changes' '
 test_expect_success 'read-tree updates worktree, absent case' '
 	echo sub/added >.git/info/sparse-checkout &&
 	git checkout -f top &&
-	git read-tree -m -u HEAD^ &&
+	read_tree_u_must_succeed -m -u HEAD^ &&
 	test ! -f init.t
 '
 
@@ -183,7 +184,7 @@ test_expect_success 'read-tree updates worktree, dirty case' '
 	echo sub/added >.git/info/sparse-checkout &&
 	git checkout -f top &&
 	echo dirty >init.t &&
-	git read-tree -m -u HEAD^ &&
+	read_tree_u_must_succeed -m -u HEAD^ &&
 	grep -q dirty init.t &&
 	rm init.t
 '
@@ -192,14 +193,14 @@ test_expect_success 'read-tree removes worktree, dirty case' '
 	echo init.t >.git/info/sparse-checkout &&
 	git checkout -f top &&
 	echo dirty >added &&
-	git read-tree -m -u HEAD^ &&
+	read_tree_u_must_succeed -m -u HEAD^ &&
 	grep -q dirty added
 '
 
 test_expect_success 'read-tree adds to worktree, absent case' '
 	echo init.t >.git/info/sparse-checkout &&
 	git checkout -f removed &&
-	git read-tree -u -m HEAD^ &&
+	read_tree_u_must_succeed -u -m HEAD^ &&
 	test ! -f sub/added
 '
 
@@ -208,7 +209,7 @@ test_expect_success 'read-tree adds to worktree, dirty case' '
 	git checkout -f removed &&
 	mkdir sub &&
 	echo dirty >sub/added &&
-	git read-tree -u -m HEAD^ &&
+	read_tree_u_must_succeed -u -m HEAD^ &&
 	grep -q dirty sub/added
 '
 
diff --git a/t/t1012-read-tree-df.sh b/t/t1012-read-tree-df.sh
index 9811d467da..a6a04b6b90 100755
--- a/t/t1012-read-tree-df.sh
+++ b/t/t1012-read-tree-df.sh
@@ -3,6 +3,7 @@
 test_description='read-tree D/F conflict corner cases'
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-read-tree.sh
 
 maketree () {
 	(
@@ -53,7 +54,7 @@ test_expect_success setup '
 
 test_expect_success '3-way (1)' '
 	settree A-000 &&
-	git read-tree -m -u O-000 A-000 B-000 &&
+	read_tree_u_must_succeed -m -u O-000 A-000 B-000 &&
 	checkindex <<-EOF
 	3 a/b
 	0 a/b-2/c/d
@@ -65,7 +66,7 @@ test_expect_success '3-way (1)' '
 
 test_expect_success '3-way (2)' '
 	settree A-001 &&
-	git read-tree -m -u O-000 A-001 B-000 &&
+	read_tree_u_must_succeed -m -u O-000 A-001 B-000 &&
 	checkindex <<-EOF
 	3 a/b
 	0 a/b-2/c/d
@@ -78,7 +79,7 @@ test_expect_success '3-way (2)' '
 
 test_expect_success '3-way (3)' '
 	settree A-010 &&
-	git read-tree -m -u O-010 A-010 B-010 &&
+	read_tree_u_must_succeed -m -u O-010 A-010 B-010 &&
 	checkindex <<-EOF
 	2 t
 	1 t-0
@@ -92,7 +93,7 @@ test_expect_success '3-way (3)' '
 
 test_expect_success '2-way (1)' '
 	settree O-020 &&
-	git read-tree -m -u O-020 A-020 &&
+	read_tree_u_must_succeed -m -u O-020 A-020 &&
 	checkindex <<-EOF
 	0 ds/dma/ioat/Makefile
 	0 ds/dma/ioat/registers.h
diff --git a/t/t1020-subdirectory.sh b/t/t1020-subdirectory.sh
index ddc3921ac6..f6a44c9ee0 100755
--- a/t/t1020-subdirectory.sh
+++ b/t/t1020-subdirectory.sh
@@ -7,6 +7,7 @@ test_description='Try various core-level commands in subdirectory.
 '
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-read-tree.sh
 
 test_expect_success setup '
 	long="a b c d e f g h i j k l m n o p q r s t u v w x y z" &&
@@ -98,13 +99,13 @@ test_expect_success 'checkout-index' '
 test_expect_success 'read-tree' '
 	rm -f one dir/two &&
 	tree=`git write-tree` &&
-	git read-tree --reset -u "$tree" &&
+	read_tree_u_must_succeed --reset -u "$tree" &&
 	cmp one original.one &&
 	cmp dir/two original.two &&
 	(
 		cd dir &&
 		rm -f two &&
-		git read-tree --reset -u "$tree" &&
+		read_tree_u_must_succeed --reset -u "$tree" &&
 		cmp two ../original.two &&
 		cmp ../one ../original.one
 	)
diff --git a/unpack-trees.c b/unpack-trees.c
index 0bc4b2ddca..07f8364244 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -203,7 +203,7 @@ static int check_updates(struct unpack_trees_options *o)
 
 		if (ce->ce_flags & CE_WT_REMOVE) {
 			display_progress(progress, ++cnt);
-			if (o->update)
+			if (o->update && !o->dry_run)
 				unlink_entry(ce);
 			continue;
 		}
@@ -217,7 +217,7 @@ static int check_updates(struct unpack_trees_options *o)
 		if (ce->ce_flags & CE_UPDATE) {
 			display_progress(progress, ++cnt);
 			ce->ce_flags &= ~CE_UPDATE;
-			if (o->update) {
+			if (o->update && !o->dry_run) {
 				errs |= checkout_entry(ce, &state, NULL);
 			}
 		}
diff --git a/unpack-trees.h b/unpack-trees.h
index cd11a08365..64f02cb03a 100644
--- a/unpack-trees.h
+++ b/unpack-trees.h
@@ -46,7 +46,8 @@ struct unpack_trees_options {
 		     debug_unpack,
 		     skip_sparse_checkout,
 		     gently,
-		     show_all_errors;
+		     show_all_errors,
+		     dry_run;
 	const char *prefix;
 	int cache_bottom;
 	struct dir_struct *dir;