win32: special-case `ENOSPC` when writing to a pipe

From: Johannes Schindelin <[email protected]>

Since c6d3cce6f3c4 (pipe_command(): handle ENOSPC when writing to a pipe, 2022-08-17), one write() call that results in an errno value ENOSPC (which typically indicates out of disk space, which makes little sense in the context of a pipe) is treated the same as EAGAIN.

However, contrary to expectations, as diagnosed in
https://github.com/python/cpython/issues/101881#issuecomment-1428667015,
when writing to a non-blocking pipe on Windows, an errno value of ENOSPC means something else: the write _fails_. Completely. Because more data was provided than the internal pipe buffer can handle. Somewhat surprising, considering that write() is allowed to write less than the specified amount, e.g. by writing only as much as fits in that buffer. But it doesn't, it writes no byte at all in that instance.

Let's handle this by manually detecting when an ENOSPC indicates that a pipe's buffer is smaller than what needs to be written, and re-try using the pipe's buffer size as size parameter.

It would be plausible to try writing the entire buffer in a loop, feeding pipe buffer-sized chunks, but experiments show that trying to write more than one buffer-sized chunk right after that will immediately fail because the buffer is unlikely to be drained as fast as write() could write again. And the whole point of a non-blocking pipe is to be non-blocking.

Which means that the logic that determines the pipe's buffer size unfortunately has to be run potentially many times when writing large amounts of data to a non-blocking pipe, as there is no elegant way to cache that information between write() calls. It's the best we can do, though, so it has to be good enough.

This fix is required to let t3701.60 (handle very large filtered diff) pass with the MSYS2 runtime provided by the MSYS2 project: Without this patch, the failed write would result in an infinite loop. This patch is not required with Git for Windows' variant of the MSYS2 runtime only because Git for Windows added an ugly work-around specifically to avoid a hang in that test case.

The diff is slightly chatty because it extends an already-existing conditional that special-cases a _different_ errno value for pipes, and because this patch needs to account for the fact that _get_osfhandle() potentially overwrites errno.

---
win32: special-case ENOSPC when writing to a pipe

This is part of my (slow, after-hours) effort to chip away at the deviations between Git for Windows' variant of the MSYS2 runtime and MSYS2's variant.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1648%2Fdscho%2Fwin32-pipe-enospc-fix-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1648/dscho/win32-pipe-enospc-fix-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1648

 compat/mingw.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

index 238a84ddbaa..320fb99a90e 100644
@@ -707,13 +707,24 @@ ssize_t mingw_write(int fd, const void *buf, size_t len)
 {
 	ssize_t result = write(fd, buf, len);
 
-	if (result < 0 && errno == EINVAL && buf) {
+	if (result < 0 && (errno == EINVAL || errno == ENOSPC) && buf) {
+		int orig = errno;
+
 		/* check if fd is a pipe */
 		HANDLE h = (HANDLE) _get_osfhandle(fd);
-		if (GetFileType(h) == FILE_TYPE_PIPE)
+		if (GetFileType(h) != FILE_TYPE_PIPE)
+			errno = orig;
+		else if (orig == EINVAL)
 			errno = EPIPE;
-		else
-			errno = EINVAL;
+		else {
+			DWORD buf_size;
+
+			if (!GetNamedPipeInfo(h, NULL, NULL, &buf_size, NULL))
+				buf_size = 4096;
+			if (len > buf_size)
+				return write(fd, buf, buf_size);
+			errno = orig;
+		}
 	}
 
 	return result;

base-commit: c5b454771e6b086f60c7f1f139025f174bcedac9

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