[PATCH v2 1/6] compat: add function to enable nonblocking pipes

From: Jeff King <peff@peff.net>
Date: Aug 17, 2022, 06:04 UTC
Message-ID: <YvyFBzdO8PN7Ou0W@coredump.intra.peff.net>
In-Reply-To: <YvyE1RDFVAg4uQo1@coredump.intra.peff.net>

We'd like to be able to make some of our pipes nonblocking so that poll() can be used effectively, but O_NONBLOCK isn't portable. Let's introduce a compat wrapper so this can be abstracted for each platform.

The interface is as narrow as possible to let platforms do what's natural there (rather than having to implement fcntl() and a fake O_NONBLOCK for example, or having to handle other types of descriptors).

The next commit will add Windows support, at which point we should be covering all platforms in practice. But if we do find some other platform without O_NONBLOCK, we'll return ENOSYS. Arguably we could just trigger a build-time #error in this case, which would catch the problem earlier. But since we're not planning to use this compat wrapper in many code paths, a seldom-seen runtime error may be friendlier for such a platform than blocking compilation completely. Our test suite would still notice it.

Signed-off-by: Jeff King <peff@peff.net>
---
 Makefile          |  1 +
 compat/nonblock.c | 23 +++++++++++++++++++++++
 compat/nonblock.h |  9 +++++++++
 3 files changed, 33 insertions(+)
 create mode 100644 compat/nonblock.c
 create mode 100644 compat/nonblock.h

diff --git a/Makefile b/Makefile
index e8adeb09f1..224e193b66 100644
--- a/Makefile
+++ b/Makefile
@@ -918,6 +918,7 @@ LIB_OBJS += combine-diff.o
 LIB_OBJS += commit-graph.o
 LIB_OBJS += commit-reach.o
 LIB_OBJS += commit.o
+LIB_OBJS += compat/nonblock.o
 LIB_OBJS += compat/obstack.o
 LIB_OBJS += compat/terminal.o
 LIB_OBJS += compat/zlib-uncompress2.o
diff --git a/compat/nonblock.c b/compat/nonblock.c
new file mode 100644
index 0000000000..b08105a21d
--- /dev/null
+++ b/compat/nonblock.c
@@ -0,0 +1,23 @@
+#include "git-compat-util.h"
+#include "nonblock.h"
+
+#ifdef O_NONBLOCK
+
+int enable_pipe_nonblock(int fd)
+{
+	int flags = fcntl(fd, F_GETFL);
+	if (flags < 0)
+		return -1;
+	flags |= O_NONBLOCK;
+	return fcntl(fd, F_SETFL, flags);
+}
+
+#else
+
+int enable_pipe_nonblock(int fd)
+{
+	errno = ENOSYS;
+	return -1;
+}
+
+#endif
diff --git a/compat/nonblock.h b/compat/nonblock.h
new file mode 100644
index 0000000000..af1a331301
--- /dev/null
+++ b/compat/nonblock.h
@@ -0,0 +1,9 @@
+#ifndef COMPAT_NONBLOCK_H
+#define COMPAT_NONBLOCK_H
+
+/*
+ * Enable non-blocking I/O for the pipe specified by the passed-in descriptor.
+ */
+int enable_pipe_nonblock(int fd);
+
+#endif

-- 
2.37.2.881.gb57357660c