[PATCH] simple-ipc: work around issues with Cygwin's Unix socket emulation

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

Cygwin emulates Unix sockets by writing files with custom contents and
then marking them as system files.

The tricky problem is that while the file is written and its `system`
bit is set, it is still identified as a file. This caused test failures
when Git is too fast looking for the Unix sockets and then complains
that there is a plain file in the way.

Let's work around this by adding a delayed retry loop, specifically for
Cygwin.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
    simple-ipc: work around issues with Cygwin's Unix socket emulation
    
    Adam Dinwoodie reported problems running the simple-ipc tests on Cygwin
    [https://lore.kernel.org/git/20211104194619.GA12886@dinwoodie.org]. This
    patch works around the underlying problem, which is rooted in Cygwin's
    implementation details.
    
    With this patch, I could not reproduce the problem, even with sh
    t0052-simple-ipc.sh --stress-limit=30.
    
    As per Junio's encouragement
    [https://lore.kernel.org/git/xmqqee7ozyx4.fsf@gitster.g], I am
    submitting this still in the -rc phase, hoping that it will make it into
    v2.34.0 final.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1074%2Fdscho%2Fcygwin-vs-simple-ipc-workaround-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1074/dscho/cygwin-vs-simple-ipc-workaround-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1074

 compat/simple-ipc/ipc-unix-socket.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/compat/simple-ipc/ipc-unix-socket.c b/compat/simple-ipc/ipc-unix-socket.c
index 4e28857a0a1..28a79289d4f 100644
--- a/compat/simple-ipc/ipc-unix-socket.c
+++ b/compat/simple-ipc/ipc-unix-socket.c
@@ -35,6 +35,28 @@ enum ipc_active_state ipc_get_active_state(const char *path)
 		}
 	}
 
+#ifdef __CYGWIN__
+	/*
+	 * Cygwin emulates Unix sockets by writing special-crafted files whose
+	 * `system` bit is set.
+	 *
+	 * If we are too fast, Cygwin might still be in the process of marking
+	 * the underlying file as a system file. Until then, we will not see a
+	 * Unix socket here, but a plain file instead. Just in case that this
+	 * is happening, wait a little and try again.
+	 */
+	{
+		static const int delay[] = { 1, 10, 20, 40, -1 };
+		int i;
+
+		for (i = 0; S_ISREG(st.st_mode) && delay[i] > 0; i++) {
+			sleep_millisec(delay[i]);
+			if (lstat(path, &st) == -1)
+				return IPC_STATE__INVALID_PATH;
+		}
+	}
+#endif
+
 	/* also complain if a plain file is in the way */
 	if ((st.st_mode & S_IFMT) != S_IFSOCK)
 		return IPC_STATE__INVALID_PATH;

base-commit: 6c220937e2b26d85920bf2d38ff2464a0d57fd6b
-- 
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