Merge branch 'ls/checkout-encoding'

The new "checkout-encoding" attribute can ask Git to convert the
contents to the specified encoding when checking out to the working
tree (and the other way around when checking in).

* ls/checkout-encoding:
  convert: add round trip check based on 'core.checkRoundtripEncoding'
  convert: add tracing for 'working-tree-encoding' attribute
  convert: check for detectable errors in UTF encodings
  convert: add 'working-tree-encoding' attribute
  utf8: add function to detect a missing UTF-16/32 BOM
  utf8: add function to detect prohibited UTF-16/32 BOM
  utf8: teach same_encoding() alternative UTF encoding names
  strbuf: add a case insensitive starts_with()
  strbuf: add xstrdup_toupper()
  strbuf: remove unnecessary NUL assignment in xstrdup_tolower()

13 files changed, 737 insertions, 5 deletions

diff --git a/Documentation/config.txt b/Documentation/config.txt
index cb4c93f74a..8213c1236b 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -530,6 +530,12 @@ core.autocrlf::
 	This variable can be set to 'input',
 	in which case no output conversion is performed.
 
+core.checkRoundtripEncoding::
+	A comma and/or whitespace separated list of encodings that Git
+	performs UTF-8 round trip checks on if they are used in an
+	`working-tree-encoding` attribute (see linkgit:gitattributes[5]).
+	The default value is `SHIFT-JIS`.
+
 core.symlinks::
 	If false, symbolic links are checked out as small plain files that
 	contain the link text. linkgit:git-update-index[1] and
diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index 1094fe2b5b..ee210be3ec 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -279,6 +279,94 @@ few exceptions.  Even though...
   catch potential problems early, safety triggers.
 
 
+`working-tree-encoding`
+^^^^^^^^^^^^^^^^^^^^^^^
+
+Git recognizes files encoded in ASCII or one of its supersets (e.g.
+UTF-8, ISO-8859-1, ...) as text files. Files encoded in certain other
+encodings (e.g. UTF-16) are interpreted as binary and consequently
+built-in Git text processing tools (e.g. 'git diff') as well as most Git
+web front ends do not visualize the contents of these files by default.
+
+In these cases you can tell Git the encoding of a file in the working
+directory with the `working-tree-encoding` attribute. If a file with this
+attribute is added to Git, then Git reencodes the content from the
+specified encoding to UTF-8. Finally, Git stores the UTF-8 encoded
+content in its internal data structure (called "the index"). On checkout
+the content is reencoded back to the specified encoding.
+
+Please note that using the `working-tree-encoding` attribute may have a
+number of pitfalls:
+
+- Alternative Git implementations (e.g. JGit or libgit2) and older Git
+  versions (as of March 2018) do not support the `working-tree-encoding`
+  attribute. If you decide to use the `working-tree-encoding` attribute
+  in your repository, then it is strongly recommended to ensure that all
+  clients working with the repository support it.
+
+  For example, Microsoft Visual Studio resources files (`*.rc`) or
+  PowerShell script files (`*.ps1`) are sometimes encoded in UTF-16.
+  If you declare `*.ps1` as files as UTF-16 and you add `foo.ps1` with
+  a `working-tree-encoding` enabled Git client, then `foo.ps1` will be
+  stored as UTF-8 internally. A client without `working-tree-encoding`
+  support will checkout `foo.ps1` as UTF-8 encoded file. This will
+  typically cause trouble for the users of this file.
+
+  If a Git client, that does not support the `working-tree-encoding`
+  attribute, adds a new file `bar.ps1`, then `bar.ps1` will be
+  stored "as-is" internally (in this example probably as UTF-16).
+  A client with `working-tree-encoding` support will interpret the
+  internal contents as UTF-8 and try to convert it to UTF-16 on checkout.
+  That operation will fail and cause an error.
+
+- Reencoding content to non-UTF encodings can cause errors as the
+  conversion might not be UTF-8 round trip safe. If you suspect your
+  encoding to not be round trip safe, then add it to
+  `core.checkRoundtripEncoding` to make Git check the round trip
+  encoding (see linkgit:git-config[1]). SHIFT-JIS (Japanese character
+  set) is known to have round trip issues with UTF-8 and is checked by
+  default.
+
+- Reencoding content requires resources that might slow down certain
+  Git operations (e.g 'git checkout' or 'git add').
+
+Use the `working-tree-encoding` attribute only if you cannot store a file
+in UTF-8 encoding and if you want Git to be able to process the content
+as text.
+
+As an example, use the following attributes if your '*.ps1' files are
+UTF-16 encoded with byte order mark (BOM) and you want Git to perform
+automatic line ending conversion based on your platform.
+
+------------------------
+*.ps1		text working-tree-encoding=UTF-16
+------------------------
+
+Use the following attributes if your '*.ps1' files are UTF-16 little
+endian encoded without BOM and you want Git to use Windows line endings
+in the working directory. Please note, it is highly recommended to
+explicitly define the line endings with `eol` if the `working-tree-encoding`
+attribute is used to avoid ambiguity.
+
+------------------------
+*.ps1		text working-tree-encoding=UTF-16LE eol=CRLF
+------------------------
+
+You can get a list of all available encodings on your platform with the
+following command:
+
+------------------------
+iconv --list
+------------------------
+
+If you do not know the encoding of a file, then you can use the `file`
+command to guess the encoding:
+
+------------------------
+file foo.ps1
+------------------------
+
+
 `ident`
 ^^^^^^^
 
diff --git a/config.c b/config.c
index 40d46b3325..9f8ca18f7b 100644
--- a/config.c
+++ b/config.c
@@ -1239,6 +1239,11 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.checkroundtripencoding")) {
+		check_roundtrip_encoding = xstrdup(value);
+		return 0;
+	}
+
 	if (!strcmp(var, "core.notesref")) {
 		notes_ref_name = xstrdup(value);
 		return 0;
diff --git a/convert.c b/convert.c
index c480097a2a..64d0d30e08 100644
--- a/convert.c
+++ b/convert.c
@@ -7,6 +7,7 @@
 #include "sigchain.h"
 #include "pkt-line.h"
 #include "sub-process.h"
+#include "utf8.h"
 
 /*
  * convert.c - convert a file when checking it out and checking it in.
@@ -265,6 +266,241 @@ static int will_convert_lf_to_crlf(size_t len, struct text_stat *stats,
 
 }
 
+static int validate_encoding(const char *path, const char *enc,
+		      const char *data, size_t len, int die_on_error)
+{
+	/* We only check for UTF here as UTF?? can be an alias for UTF-?? */
+	if (istarts_with(enc, "UTF")) {
+		/*
+		 * Check for detectable errors in UTF encodings
+		 */
+		if (has_prohibited_utf_bom(enc, data, len)) {
+			const char *error_msg = _(
+				"BOM is prohibited in '%s' if encoded as %s");
+			/*
+			 * This advice is shown for UTF-??BE and UTF-??LE encodings.
+			 * We cut off the last two characters of the encoding name
+			 * to generate the encoding name suitable for BOMs.
+			 */
+			const char *advise_msg = _(
+				"The file '%s' contains a byte order "
+				"mark (BOM). Please use UTF-%s as "
+				"working-tree-encoding.");
+			const char *stripped = NULL;
+			char *upper = xstrdup_toupper(enc);
+			upper[strlen(upper)-2] = '\0';
+			if (!skip_prefix(upper, "UTF-", &stripped))
+				skip_prefix(stripped, "UTF", &stripped);
+			advise(advise_msg, path, stripped);
+			free(upper);
+			if (die_on_error)
+				die(error_msg, path, enc);
+			else {
+				return error(error_msg, path, enc);
+			}
+
+		} else if (is_missing_required_utf_bom(enc, data, len)) {
+			const char *error_msg = _(
+				"BOM is required in '%s' if encoded as %s");
+			const char *advise_msg = _(
+				"The file '%s' is missing a byte order "
+				"mark (BOM). Please use UTF-%sBE or UTF-%sLE "
+				"(depending on the byte order) as "
+				"working-tree-encoding.");
+			const char *stripped = NULL;
+			char *upper = xstrdup_toupper(enc);
+			if (!skip_prefix(upper, "UTF-", &stripped))
+				skip_prefix(stripped, "UTF", &stripped);
+			advise(advise_msg, path, stripped, stripped);
+			free(upper);
+			if (die_on_error)
+				die(error_msg, path, enc);
+			else {
+				return error(error_msg, path, enc);
+			}
+		}
+
+	}
+	return 0;
+}
+
+static void trace_encoding(const char *context, const char *path,
+			   const char *encoding, const char *buf, size_t len)
+{
+	static struct trace_key coe = TRACE_KEY_INIT(WORKING_TREE_ENCODING);
+	struct strbuf trace = STRBUF_INIT;
+	int i;
+
+	strbuf_addf(&trace, "%s (%s, considered %s):\n", context, path, encoding);
+	for (i = 0; i < len && buf; ++i) {
+		strbuf_addf(
+			&trace,"| \e[2m%2i:\e[0m %2x \e[2m%c\e[0m%c",
+			i,
+			(unsigned char) buf[i],
+			(buf[i] > 32 && buf[i] < 127 ? buf[i] : ' '),
+			((i+1) % 8 && (i+1) < len ? ' ' : '\n')
+		);
+	}
+	strbuf_addchars(&trace, '\n', 1);
+
+	trace_strbuf(&coe, &trace);
+	strbuf_release(&trace);
+}
+
+static int check_roundtrip(const char *enc_name)
+{
+	/*
+	 * check_roundtrip_encoding contains a string of comma and/or
+	 * space separated encodings (eg. "UTF-16, ASCII, CP1125").
+	 * Search for the given encoding in that string.
+	 */
+	const char *found = strcasestr(check_roundtrip_encoding, enc_name);
+	const char *next;
+	int len;
+	if (!found)
+		return 0;
+	next = found + strlen(enc_name);
+	len = strlen(check_roundtrip_encoding);
+	return (found && (
+			/*
+			 * check that the found encoding is at the
+			 * beginning of check_roundtrip_encoding or
+			 * that it is prefixed with a space or comma
+			 */
+			found == check_roundtrip_encoding || (
+				(isspace(found[-1]) || found[-1] == ',')
+			)
+		) && (
+			/*
+			 * check that the found encoding is at the
+			 * end of check_roundtrip_encoding or
+			 * that it is suffixed with a space or comma
+			 */
+			next == check_roundtrip_encoding + len || (
+				next < check_roundtrip_encoding + len &&
+				(isspace(next[0]) || next[0] == ',')
+			)
+		));
+}
+
+static const char *default_encoding = "UTF-8";
+
+static int encode_to_git(const char *path, const char *src, size_t src_len,
+			 struct strbuf *buf, const char *enc, int conv_flags)
+{
+	char *dst;
+	int dst_len;
+	int die_on_error = conv_flags & CONV_WRITE_OBJECT;
+
+	/*
+	 * No encoding is specified or there is nothing to encode.
+	 * Tell the caller that the content was not modified.
+	 */
+	if (!enc || (src && !src_len))
+		return 0;
+
+	/*
+	 * Looks like we got called from "would_convert_to_git()".
+	 * This means Git wants to know if it would encode (= modify!)
+	 * the content. Let's answer with "yes", since an encoding was
+	 * specified.
+	 */
+	if (!buf && !src)
+		return 1;
+
+	if (validate_encoding(path, enc, src, src_len, die_on_error))
+		return 0;
+
+	trace_encoding("source", path, enc, src, src_len);
+	dst = reencode_string_len(src, src_len, default_encoding, enc,
+				  &dst_len);
+	if (!dst) {
+		/*
+		 * We could add the blob "as-is" to Git. However, on checkout
+		 * we would try to reencode to the original encoding. This
+		 * would fail and we would leave the user with a messed-up
+		 * working tree. Let's try to avoid this by screaming loud.
+		 */
+		const char* msg = _("failed to encode '%s' from %s to %s");
+		if (die_on_error)
+			die(msg, path, enc, default_encoding);
+		else {
+			error(msg, path, enc, default_encoding);
+			return 0;
+		}
+	}
+	trace_encoding("destination", path, default_encoding, dst, dst_len);
+
+	/*
+	 * UTF supports lossless conversion round tripping [1] and conversions
+	 * between UTF and other encodings are mostly round trip safe as
+	 * Unicode aims to be a superset of all other character encodings.
+	 * However, certain encodings (e.g. SHIFT-JIS) are known to have round
+	 * trip issues [2]. Check the round trip conversion for all encodings
+	 * listed in core.checkRoundtripEncoding.
+	 *
+	 * The round trip check is only performed if content is written to Git.
+	 * This ensures that no information is lost during conversion to/from
+	 * the internal UTF-8 representation.
+	 *
+	 * Please note, the code below is not tested because I was not able to
+	 * generate a faulty round trip without an iconv error. Iconv errors
+	 * are already caught above.
+	 *
+	 * [1] http://unicode.org/faq/utf_bom.html#gen2
+	 * [2] https://support.microsoft.com/en-us/help/170559/prb-conversion-problem-between-shift-jis-and-unicode
+	 */
+	if (die_on_error && check_roundtrip(enc)) {
+		char *re_src;
+		int re_src_len;
+
+		re_src = reencode_string_len(dst, dst_len,
+					     enc, default_encoding,
+					     &re_src_len);
+
+		trace_printf("Checking roundtrip encoding for %s...\n", enc);
+		trace_encoding("reencoded source", path, enc,
+			       re_src, re_src_len);
+
+		if (!re_src || src_len != re_src_len ||
+		    memcmp(src, re_src, src_len)) {
+			const char* msg = _("encoding '%s' from %s to %s and "
+					    "back is not the same");
+			die(msg, path, enc, default_encoding);
+		}
+
+		free(re_src);
+	}
+
+	strbuf_attach(buf, dst, dst_len, dst_len + 1);
+	return 1;
+}
+
+static int encode_to_worktree(const char *path, const char *src, size_t src_len,
+			      struct strbuf *buf, const char *enc)
+{
+	char *dst;
+	int dst_len;
+
+	/*
+	 * No encoding is specified or there is nothing to encode.
+	 * Tell the caller that the content was not modified.
+	 */
+	if (!enc || (src && !src_len))
+		return 0;
+
+	dst = reencode_string_len(src, src_len, enc, default_encoding,
+				  &dst_len);
+	if (!dst) {
+		error("failed to encode '%s' from %s to %s",
+			path, default_encoding, enc);
+		return 0;
+	}
+
+	strbuf_attach(buf, dst, dst_len, dst_len + 1);
+	return 1;
+}
+
 static int crlf_to_git(const struct index_state *istate,
 		       const char *path, const char *src, size_t len,
 		       struct strbuf *buf,
@@ -978,6 +1214,24 @@ static int ident_to_worktree(const char *path, const char *src, size_t len,
 	return 1;
 }
 
+static const char *git_path_check_encoding(struct attr_check_item *check)
+{
+	const char *value = check->value;
+
+	if (ATTR_UNSET(value) || !strlen(value))
+		return NULL;
+
+	if (ATTR_TRUE(value) || ATTR_FALSE(value)) {
+		die(_("true/false are no valid working-tree-encodings"));
+	}
+
+	/* Don't encode to the default encoding */
+	if (same_encoding(value, default_encoding))
+		return NULL;
+
+	return value;
+}
+
 static enum crlf_action git_path_check_crlf(struct attr_check_item *check)
 {
 	const char *value = check->value;
@@ -1033,6 +1287,7 @@ struct conv_attrs {
 	enum crlf_action attr_action; /* What attr says */
 	enum crlf_action crlf_action; /* When no attr is set, use core.autocrlf */
 	int ident;
+	const char *working_tree_encoding; /* Supported encoding or default encoding if NULL */
 };
 
 static void convert_attrs(struct conv_attrs *ca, const char *path)
@@ -1041,7 +1296,8 @@ static void convert_attrs(struct conv_attrs *ca, const char *path)
 
 	if (!check) {
 		check = attr_check_initl("crlf", "ident", "filter",
-					 "eol", "text", NULL);
+					 "eol", "text", "working-tree-encoding",
+					 NULL);
 		user_convert_tail = &user_convert;
 		git_config(read_convert_config, NULL);
 	}
@@ -1064,6 +1320,7 @@ static void convert_attrs(struct conv_attrs *ca, const char *path)
 			else if (eol_attr == EOL_CRLF)
 				ca->crlf_action = CRLF_TEXT_CRLF;
 		}
+		ca->working_tree_encoding = git_path_check_encoding(ccheck + 5);
 	} else {
 		ca->drv = NULL;
 		ca->crlf_action = CRLF_UNDEFINED;
@@ -1144,6 +1401,13 @@ int convert_to_git(const struct index_state *istate,
 		src = dst->buf;
 		len = dst->len;
 	}
+
+	ret |= encode_to_git(path, src, len, dst, ca.working_tree_encoding, conv_flags);
+	if (ret && dst) {
+		src = dst->buf;
+		len = dst->len;
+	}
+
 	if (!(conv_flags & CONV_EOL_KEEP_CRLF)) {
 		ret |= crlf_to_git(istate, path, src, len, dst, ca.crlf_action, conv_flags);
 		if (ret && dst) {
@@ -1167,6 +1431,7 @@ void convert_to_git_filter_fd(const struct index_state *istate,
 	if (!apply_filter(path, NULL, 0, fd, dst, ca.drv, CAP_CLEAN, NULL))
 		die("%s: clean filter '%s' failed", path, ca.drv->name);
 
+	encode_to_git(path, dst->buf, dst->len, dst, ca.working_tree_encoding, conv_flags);
 	crlf_to_git(istate, path, dst->buf, dst->len, dst, ca.crlf_action, conv_flags);
 	ident_to_git(path, dst->buf, dst->len, dst, ca.ident);
 }
@@ -1198,6 +1463,12 @@ static int convert_to_working_tree_internal(const char *path, const char *src,
 		}
 	}
 
+	ret |= encode_to_worktree(path, src, len, dst, ca.working_tree_encoding);
+	if (ret) {
+		src = dst->buf;
+		len = dst->len;
+	}
+
 	ret_filter = apply_filter(
 		path, src, len, -1, dst, ca.drv, CAP_SMUDGE, dco);
 	if (!ret_filter && ca.drv && ca.drv->required)
@@ -1664,6 +1935,9 @@ struct stream_filter *get_stream_filter(const char *path, const struct object_id
 	if (ca.drv && (ca.drv->process || ca.drv->smudge || ca.drv->clean))
 		return NULL;
 
+	if (ca.working_tree_encoding)
+		return NULL;
+
 	if (ca.crlf_action == CRLF_AUTO || ca.crlf_action == CRLF_AUTO_CRLF)
 		return NULL;
 
diff --git a/convert.h b/convert.h
index 2e9b4f49cc..01385d9288 100644
--- a/convert.h
+++ b/convert.h
@@ -12,6 +12,7 @@ struct index_state;
 #define CONV_EOL_RNDTRP_WARN  (1<<1) /* Warn if CRLF to LF to CRLF is different */
 #define CONV_EOL_RENORMALIZE  (1<<2) /* Convert CRLF to LF */
 #define CONV_EOL_KEEP_CRLF    (1<<3) /* Keep CRLF line endings as is */
+#define CONV_WRITE_OBJECT     (1<<4) /* Content is written to the index */
 
 extern int global_conv_flags_eol;
 
@@ -55,6 +56,7 @@ struct delayed_checkout {
 };
 
 extern enum eol core_eol;
+extern char *check_roundtrip_encoding;
 extern const char *get_cached_convert_stats_ascii(const struct index_state *istate,
 						  const char *path);
 extern const char *get_wt_convert_stats_ascii(const char *path);
diff --git a/environment.c b/environment.c
index f4dd45eb3d..2a6de2330b 100644
--- a/environment.c
+++ b/environment.c
@@ -55,6 +55,7 @@ int check_replace_refs = 1; /* NEEDSWORK: rename to read_replace_refs */
 char *git_replace_ref_base;
 enum eol core_eol = EOL_UNSET;
 int global_conv_flags_eol = CONV_EOL_RNDTRP_WARN;
+char *check_roundtrip_encoding = "SHIFT-JIS";
 unsigned whitespace_rule_cfg = WS_DEFAULT_RULE;
 enum branch_track git_branch_track = BRANCH_TRACK_REMOTE;
 enum rebase_setup_type autorebase = AUTOREBASE_NEVER;
diff --git a/git-compat-util.h b/git-compat-util.h
index 07e383257b..f1d7ada890 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -455,6 +455,7 @@ extern void (*get_warn_routine(void))(const char *warn, va_list params);
 extern void set_die_is_recursing_routine(int (*routine)(void));
 
 extern int starts_with(const char *str, const char *prefix);
+extern int istarts_with(const char *str, const char *prefix);
 
 /*
  * If the string "str" begins with the string found in "prefix", return 1.
diff --git a/sha1-file.c b/sha1-file.c
index 64a5bd7d87..46072602ff 100644
--- a/sha1-file.c
+++ b/sha1-file.c
@@ -142,7 +142,7 @@ static int get_conv_flags(unsigned flags)
 	if (flags & HASH_RENORMALIZE)
 		return CONV_EOL_RENORMALIZE;
 	else if (flags & HASH_WRITE_OBJECT)
-	  return global_conv_flags_eol;
+		return global_conv_flags_eol | CONV_WRITE_OBJECT;
 	else
 		return 0;
 }
diff --git a/strbuf.c b/strbuf.c
index 43a840c67b..622c462d54 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -11,6 +11,15 @@ int starts_with(const char *str, const char *prefix)
 			return 0;
 }
 
+int istarts_with(const char *str, const char *prefix)
+{
+	for (; ; str++, prefix++)
+		if (!*prefix)
+			return 1;
+		else if (tolower(*str) != tolower(*prefix))
+			return 0;
+}
+
 int skip_to_optional_arg_default(const char *str, const char *prefix,
 				 const char **arg, const char *def)
 {
@@ -793,7 +802,18 @@ char *xstrdup_tolower(const char *string)
 	result = xmallocz(len);
 	for (i = 0; i < len; i++)
 		result[i] = tolower(string[i]);
-	result[i] = '\0';
+	return result;
+}
+
+char *xstrdup_toupper(const char *string)
+{
+	char *result;
+	size_t len, i;
+
+	len = strlen(string);
+	result = xmallocz(len);
+	for (i = 0; i < len; i++)
+		result[i] = toupper(string[i]);
 	return result;
 }
 
diff --git a/strbuf.h b/strbuf.h
index 4efa80c1de..8c25e4bb59 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -616,6 +616,7 @@ __attribute__((format (printf,2,3)))
 extern int fprintf_ln(FILE *fp, const char *fmt, ...);
 
 char *xstrdup_tolower(const char *);
+char *xstrdup_toupper(const char *);
 
 /**
  * Create a newly allocated string using printf format. You can do this easily
diff --git a/t/t0028-working-tree-encoding.sh b/t/t0028-working-tree-encoding.sh
new file mode 100755
index 0000000000..12b8eb963a
--- /dev/null
+++ b/t/t0028-working-tree-encoding.sh
@@ -0,0 +1,245 @@
+#!/bin/sh
+
+test_description='working-tree-encoding conversion via gitattributes'
+
+. ./test-lib.sh
+
+GIT_TRACE_WORKING_TREE_ENCODING=1 && export GIT_TRACE_WORKING_TREE_ENCODING
+
+test_expect_success 'setup test files' '
+	git config core.eol lf &&
+
+	text="hallo there!\ncan you read me?" &&
+	echo "*.utf16 text working-tree-encoding=utf-16" >.gitattributes &&
+	printf "$text" >test.utf8.raw &&
+	printf "$text" | iconv -f UTF-8 -t UTF-16 >test.utf16.raw &&
+	printf "$text" | iconv -f UTF-8 -t UTF-32 >test.utf32.raw &&
+
+	# Line ending tests
+	printf "one\ntwo\nthree\n" >lf.utf8.raw &&
+	printf "one\r\ntwo\r\nthree\r\n" >crlf.utf8.raw &&
+
+	# BOM tests
+	printf "\0a\0b\0c"                         >nobom.utf16be.raw &&
+	printf "a\0b\0c\0"                         >nobom.utf16le.raw &&
+	printf "\376\777\0a\0b\0c"                 >bebom.utf16be.raw &&
+	printf "\777\376a\0b\0c\0"                 >lebom.utf16le.raw &&
+	printf "\0\0\0a\0\0\0b\0\0\0c"             >nobom.utf32be.raw &&
+	printf "a\0\0\0b\0\0\0c\0\0\0"             >nobom.utf32le.raw &&
+	printf "\0\0\376\777\0\0\0a\0\0\0b\0\0\0c" >bebom.utf32be.raw &&
+	printf "\777\376\0\0a\0\0\0b\0\0\0c\0\0\0" >lebom.utf32le.raw &&
+
+	# Add only UTF-16 file, we will add the UTF-32 file later
+	cp test.utf16.raw test.utf16 &&
+	cp test.utf32.raw test.utf32 &&
+	git add .gitattributes test.utf16 &&
+	git commit -m initial
+'
+
+test_expect_success 'ensure UTF-8 is stored in Git' '
+	test_when_finished "rm -f test.utf16.git" &&
+
+	git cat-file -p :test.utf16 >test.utf16.git &&
+	test_cmp_bin test.utf8.raw test.utf16.git
+'
+
+test_expect_success 're-encode to UTF-16 on checkout' '
+	test_when_finished "rm -f test.utf16.raw" &&
+
+	rm test.utf16 &&
+	git checkout test.utf16 &&
+	test_cmp_bin test.utf16.raw test.utf16
+'
+
+test_expect_success 'check $GIT_DIR/info/attributes support' '
+	test_when_finished "rm -f test.utf32.git" &&
+	test_when_finished "git reset --hard HEAD" &&
+
+	echo "*.utf32 text working-tree-encoding=utf-32" >.git/info/attributes &&
+	git add test.utf32 &&
+
+	git cat-file -p :test.utf32 >test.utf32.git &&
+	test_cmp_bin test.utf8.raw test.utf32.git
+'
+
+for i in 16 32
+do
+	test_expect_success "check prohibited UTF-${i} BOM" '
+		test_when_finished "git reset --hard HEAD" &&
+
+		echo "*.utf${i}be text working-tree-encoding=utf-${i}be" >>.gitattributes &&
+		echo "*.utf${i}le text working-tree-encoding=utf-${i}LE" >>.gitattributes &&
+
+		# Here we add a UTF-16 (resp. UTF-32) files with BOM (big/little-endian)
+		# but we tell Git to treat it as UTF-16BE/UTF-16LE (resp. UTF-32).
+		# In these cases the BOM is prohibited.
+		cp bebom.utf${i}be.raw bebom.utf${i}be &&
+		test_must_fail git add bebom.utf${i}be 2>err.out &&
+		test_i18ngrep "fatal: BOM is prohibited .* utf-${i}be" err.out &&
+		test_i18ngrep "use UTF-${i} as working-tree-encoding" err.out &&
+
+		cp lebom.utf${i}le.raw lebom.utf${i}be &&
+		test_must_fail git add lebom.utf${i}be 2>err.out &&
+		test_i18ngrep "fatal: BOM is prohibited .* utf-${i}be" err.out &&
+		test_i18ngrep "use UTF-${i} as working-tree-encoding" err.out &&
+
+		cp bebom.utf${i}be.raw bebom.utf${i}le &&
+		test_must_fail git add bebom.utf${i}le 2>err.out &&
+		test_i18ngrep "fatal: BOM is prohibited .* utf-${i}LE" err.out &&
+		test_i18ngrep "use UTF-${i} as working-tree-encoding" err.out &&
+
+		cp lebom.utf${i}le.raw lebom.utf${i}le &&
+		test_must_fail git add lebom.utf${i}le 2>err.out &&
+		test_i18ngrep "fatal: BOM is prohibited .* utf-${i}LE" err.out &&
+		test_i18ngrep "use UTF-${i} as working-tree-encoding" err.out
+	'
+
+	test_expect_success "check required UTF-${i} BOM" '
+		test_when_finished "git reset --hard HEAD" &&
+
+		echo "*.utf${i} text working-tree-encoding=utf-${i}" >>.gitattributes &&
+
+		cp nobom.utf${i}be.raw nobom.utf${i} &&
+		test_must_fail git add nobom.utf${i} 2>err.out &&
+		test_i18ngrep "fatal: BOM is required .* utf-${i}" err.out &&
+		test_i18ngrep "use UTF-${i}BE or UTF-${i}LE" err.out &&
+
+		cp nobom.utf${i}le.raw nobom.utf${i} &&
+		test_must_fail git add nobom.utf${i} 2>err.out &&
+		test_i18ngrep "fatal: BOM is required .* utf-${i}" err.out &&
+		test_i18ngrep "use UTF-${i}BE or UTF-${i}LE" err.out
+	'
+
+	test_expect_success "eol conversion for UTF-${i} encoded files on checkout" '
+		test_when_finished "rm -f crlf.utf${i}.raw lf.utf${i}.raw" &&
+		test_when_finished "git reset --hard HEAD^" &&
+
+		cat lf.utf8.raw | iconv -f UTF-8 -t UTF-${i} >lf.utf${i}.raw &&
+		cat crlf.utf8.raw | iconv -f UTF-8 -t UTF-${i} >crlf.utf${i}.raw &&
+		cp crlf.utf${i}.raw eol.utf${i} &&
+
+		cat >expectIndexLF <<-EOF &&
+			i/lf    w/-text attr/text             	eol.utf${i}
+		EOF
+
+		git add eol.utf${i} &&
+		git commit -m eol &&
+
+		# UTF-${i} with CRLF (Windows line endings)
+		rm eol.utf${i} &&
+		git -c core.eol=crlf checkout eol.utf${i} &&
+		test_cmp_bin crlf.utf${i}.raw eol.utf${i} &&
+
+		# Although the file has CRLF in the working tree,
+		# ensure LF in the index
+		git ls-files --eol eol.utf${i} >actual &&
+		test_cmp expectIndexLF actual &&
+
+		# UTF-${i} with LF (Unix line endings)
+		rm eol.utf${i} &&
+		git -c core.eol=lf checkout eol.utf${i} &&
+		test_cmp_bin lf.utf${i}.raw eol.utf${i} &&
+
+		# The file LF in the working tree, ensure LF in the index
+		git ls-files --eol eol.utf${i} >actual &&
+		test_cmp expectIndexLF actual
+	'
+done
+
+test_expect_success 'check unsupported encodings' '
+	test_when_finished "git reset --hard HEAD" &&
+
+	echo "*.set text working-tree-encoding" >.gitattributes &&
+	printf "set" >t.set &&
+	test_must_fail git add t.set 2>err.out &&
+	test_i18ngrep "true/false are no valid working-tree-encodings" err.out &&
+
+	echo "*.unset text -working-tree-encoding" >.gitattributes &&
+	printf "unset" >t.unset &&
+	git add t.unset &&
+
+	echo "*.empty text working-tree-encoding=" >.gitattributes &&
+	printf "empty" >t.empty &&
+	git add t.empty &&
+
+	echo "*.garbage text working-tree-encoding=garbage" >.gitattributes &&
+	printf "garbage" >t.garbage &&
+	test_must_fail git add t.garbage 2>err.out &&
+	test_i18ngrep "failed to encode" err.out
+'
+
+test_expect_success 'error if encoding round trip is not the same during refresh' '
+	BEFORE_STATE=$(git rev-parse HEAD) &&
+	test_when_finished "git reset --hard $BEFORE_STATE" &&
+
+	# Add and commit a UTF-16 file but skip the "working-tree-encoding"
+	# filter. Consequently, the in-repo representation is UTF-16 and not
+	# UTF-8. This simulates a Git version that has no working tree encoding
+	# support.
+	echo "*.utf16le text working-tree-encoding=utf-16le" >.gitattributes &&
+	echo "hallo" >nonsense.utf16le &&
+	TEST_HASH=$(git hash-object --no-filters -w nonsense.utf16le) &&
+	git update-index --add --cacheinfo 100644 $TEST_HASH nonsense.utf16le &&
+	COMMIT=$(git commit-tree -p $(git rev-parse HEAD) -m "plain commit" $(git write-tree)) &&
+	git update-ref refs/heads/master $COMMIT &&
+
+	test_must_fail git checkout HEAD^ 2>err.out &&
+	test_i18ngrep "error: .* overwritten by checkout:" err.out
+'
+
+test_expect_success 'error if encoding garbage is already in Git' '
+	BEFORE_STATE=$(git rev-parse HEAD) &&
+	test_when_finished "git reset --hard $BEFORE_STATE" &&
+
+	# Skip the UTF-16 filter for the added file
+	# This simulates a Git version that has no checkoutEncoding support
+	cp nobom.utf16be.raw nonsense.utf16 &&
+	TEST_HASH=$(git hash-object --no-filters -w nonsense.utf16) &&
+	git update-index --add --cacheinfo 100644 $TEST_HASH nonsense.utf16 &&
+	COMMIT=$(git commit-tree -p $(git rev-parse HEAD) -m "plain commit" $(git write-tree)) &&
+	git update-ref refs/heads/master $COMMIT &&
+
+	git diff 2>err.out &&
+	test_i18ngrep "error: BOM is required" err.out
+'
+
+test_expect_success 'check roundtrip encoding' '
+	test_when_finished "rm -f roundtrip.shift roundtrip.utf16" &&
+	test_when_finished "git reset --hard HEAD" &&
+
+	text="hallo there!\nroundtrip test here!" &&
+	printf "$text" | iconv -f UTF-8 -t SHIFT-JIS >roundtrip.shift &&
+	printf "$text" | iconv -f UTF-8 -t UTF-16 >roundtrip.utf16 &&
+	echo "*.shift text working-tree-encoding=SHIFT-JIS" >>.gitattributes &&
+
+	# SHIFT-JIS encoded files are round-trip checked by default...
+	GIT_TRACE=1 git add .gitattributes roundtrip.shift 2>&1 |
+		grep "Checking roundtrip encoding for SHIFT-JIS" &&
+	git reset &&
+
+	# ... unless we overwrite the Git config!
+	! GIT_TRACE=1 git -c core.checkRoundtripEncoding=garbage \
+		add .gitattributes roundtrip.shift 2>&1 |
+		grep "Checking roundtrip encoding for SHIFT-JIS" &&
+	git reset &&
+
+	# UTF-16 encoded files should not be round-trip checked by default...
+	! GIT_TRACE=1 git add roundtrip.utf16 2>&1 |
+		grep "Checking roundtrip encoding for UTF-16" &&
+	git reset &&
+
+	# ... unless we tell Git to check it!
+	GIT_TRACE=1 git -c core.checkRoundtripEncoding="UTF-16, UTF-32" \
+		add roundtrip.utf16 2>&1 |
+		grep "Checking roundtrip encoding for utf-16" &&
+	git reset &&
+
+	# ... unless we tell Git to check it!
+	# (here we also check that the casing of the encoding is irrelevant)
+	GIT_TRACE=1 git -c core.checkRoundtripEncoding="UTF-32, utf-16" \
+		add roundtrip.utf16 2>&1 |
+		grep "Checking roundtrip encoding for utf-16" &&
+	git reset
+'
+
+test_done
diff --git a/utf8.c b/utf8.c
index 4419055b48..0fcc6487e3 100644
--- a/utf8.c
+++ b/utf8.c
@@ -401,18 +401,40 @@ out:
 	strbuf_release(&sb_dst);
 }
 
+/*
+ * Returns true (1) if the src encoding name matches the dst encoding
+ * name directly or one of its alternative names. E.g. UTF-16BE is the
+ * same as UTF16BE.
+ */
+static int same_utf_encoding(const char *src, const char *dst)
+{
+	if (istarts_with(src, "utf") && istarts_with(dst, "utf")) {
+		/* src[3] or dst[3] might be '\0' */
+		int i = (src[3] == '-' ? 4 : 3);
+		int j = (dst[3] == '-' ? 4 : 3);
+		return !strcasecmp(src+i, dst+j);
+	}
+	return 0;
+}
+
 int is_encoding_utf8(const char *name)
 {
 	if (!name)
 		return 1;
-	if (!strcasecmp(name, "utf-8") || !strcasecmp(name, "utf8"))
+	if (same_utf_encoding("utf-8", name))
 		return 1;
 	return 0;
 }
 
 int same_encoding(const char *src, const char *dst)
 {
-	if (is_encoding_utf8(src) && is_encoding_utf8(dst))
+	static const char utf8[] = "UTF-8";
+
+	if (!src)
+		src = utf8;
+	if (!dst)
+		dst = utf8;
+	if (same_utf_encoding(src, dst))
 		return 1;
 	return !strcasecmp(src, dst);
 }
@@ -538,6 +560,45 @@ char *reencode_string_len(const char *in, int insz,
 }
 #endif
 
+static int has_bom_prefix(const char *data, size_t len,
+			  const char *bom, size_t bom_len)
+{
+	return data && bom && (len >= bom_len) && !memcmp(data, bom, bom_len);
+}
+
+static const char utf16_be_bom[] = {0xFE, 0xFF};
+static const char utf16_le_bom[] = {0xFF, 0xFE};
+static const char utf32_be_bom[] = {0x00, 0x00, 0xFE, 0xFF};
+static const char utf32_le_bom[] = {0xFF, 0xFE, 0x00, 0x00};
+
+int has_prohibited_utf_bom(const char *enc, const char *data, size_t len)
+{
+	return (
+	  (same_utf_encoding("UTF-16BE", enc) ||
+	   same_utf_encoding("UTF-16LE", enc)) &&
+	  (has_bom_prefix(data, len, utf16_be_bom, sizeof(utf16_be_bom)) ||
+	   has_bom_prefix(data, len, utf16_le_bom, sizeof(utf16_le_bom)))
+	) || (
+	  (same_utf_encoding("UTF-32BE",  enc) ||
+	   same_utf_encoding("UTF-32LE", enc)) &&
+	  (has_bom_prefix(data, len, utf32_be_bom, sizeof(utf32_be_bom)) ||
+	   has_bom_prefix(data, len, utf32_le_bom, sizeof(utf32_le_bom)))
+	);
+}
+
+int is_missing_required_utf_bom(const char *enc, const char *data, size_t len)
+{
+	return (
+	   (same_utf_encoding(enc, "UTF-16")) &&
+	   !(has_bom_prefix(data, len, utf16_be_bom, sizeof(utf16_be_bom)) ||
+	     has_bom_prefix(data, len, utf16_le_bom, sizeof(utf16_le_bom)))
+	) || (
+	   (same_utf_encoding(enc, "UTF-32")) &&
+	   !(has_bom_prefix(data, len, utf32_be_bom, sizeof(utf32_be_bom)) ||
+	     has_bom_prefix(data, len, utf32_le_bom, sizeof(utf32_le_bom)))
+	);
+}
+
 /*
  * Returns first character length in bytes for multi-byte `text` according to
  * `encoding`.
diff --git a/utf8.h b/utf8.h
index 6bbcf31a83..cce654a64a 100644
--- a/utf8.h
+++ b/utf8.h
@@ -70,4 +70,32 @@ typedef enum {
 void strbuf_utf8_align(struct strbuf *buf, align_type position, unsigned int width,
 		       const char *s);
 
+/*
+ * If a data stream is declared as UTF-16BE or UTF-16LE, then a UTF-16
+ * BOM must not be used [1]. The same applies for the UTF-32 equivalents.
+ * The function returns true if this rule is violated.
+ *
+ * [1] http://unicode.org/faq/utf_bom.html#bom10
+ */
+int has_prohibited_utf_bom(const char *enc, const char *data, size_t len);
+
+/*
+ * If the endianness is not defined in the encoding name, then we
+ * require a BOM. The function returns true if a required BOM is missing.
+ *
+ * The Unicode standard instructs to assume big-endian if there in no
+ * BOM for UTF-16/32 [1][2]. However, the W3C/WHATWG encoding standard
+ * used in HTML5 recommends to assume little-endian to "deal with
+ * deployed content" [3].
+ *
+ * Therefore, strictly requiring a BOM seems to be the safest option for
+ * content in Git.
+ *
+ * [1] http://unicode.org/faq/utf_bom.html#gen6
+ * [2] http://www.unicode.org/versions/Unicode10.0.0/ch03.pdf
+ *     Section 3.10, D98, page 132
+ * [3] https://encoding.spec.whatwg.org/#utf-16le
+ */
+int is_missing_required_utf_bom(const char *enc, const char *data, size_t len);
+
 #endif