unix-stream-server: create unix domain socket under lock

Create a wrapper class for `unix_stream_listen()` that uses a ".lock"
lockfile to create the unix domain socket in a race-free manner.

Unix domain sockets have a fundamental problem on Unix systems because
they persist in the filesystem until they are deleted.  This is
independent of whether a server is actually listening for connections.
Well-behaved servers are expected to delete the socket when they
shutdown.  A new server cannot easily tell if a found socket is
attached to an active server or is leftover cruft from a dead server.
The traditional solution used by `unix_stream_listen()` is to force
delete the socket pathname and then create a new socket.  This solves
the latter (cruft) problem, but in the case of the former, it orphans
the existing server (by stealing the pathname associated with the
socket it is listening on).

We cannot directly use a .lock lockfile to create the socket because
the socket is created by `bind(2)` rather than the `open(2)` mechanism
used by `tempfile.c`.

As an alternative, we hold a plain lockfile ("<path>.lock") as a
mutual exclusion device.  Under the lock, we test if an existing
socket ("<path>") is has an active server.  If not, we create a new
socket and begin listening.  Then we use "rollback" to delete the
lockfile in all cases.

This wrapper code conceptually exists at a higher-level than the core
unix_stream_connect() and unix_stream_listen() routines that it
consumes.  It is isolated in a wrapper class for clarity.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/contrib/buildsystems/CMakeLists.txt b/contrib/buildsystems/CMakeLists.txt
index 4bd41054ee..6ffbbb6a6f 100644
--- a/contrib/buildsystems/CMakeLists.txt
+++ b/contrib/buildsystems/CMakeLists.txt
@@ -243,7 +243,7 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
 
 elseif(CMAKE_SYSTEM_NAME STREQUAL "Linux")
 	add_compile_definitions(PROCFS_EXECUTABLE_PATH="/proc/self/exe" HAVE_DEV_TTY )
-	list(APPEND compat_SOURCES unix-socket.c)
+	list(APPEND compat_SOURCES unix-socket.c unix-stream-server.c)
 endif()
 
 if(CMAKE_SYSTEM_NAME STREQUAL "Windows")