Improve the sshnodelay hack after osxfuse's merge

Document the strage-looking symbol interposition that is necessary for the
OS X case.

While doing this, homogenize the code to do the real connect call for both
the Linux and OS X cases into a helper function.  This logic is generic
and should not be replicated into various OS-specific cases.

1 files changed, 27 insertions, 14 deletions

diff --git a/sshnodelay.c b/sshnodelay.c
index e1d9220..a9f307c 100644
--- a/sshnodelay.c
+++ b/sshnodelay.c
@@ -5,8 +5,32 @@
 #include <netinet/in.h>
 #include <netinet/tcp.h>
 
+/* Wrapper around connect(2) to explicitly set TCP_NODELAY. */
+static int nodelay_connect(
+    int (*real_connect)(int, const struct sockaddr *, socklen_t),
+    int sock, const struct sockaddr *addr, socklen_t addrlen)
+{
+	int res = real_connect(sock, addr, addrlen);
+	if (!res && addr->sa_family == AF_INET) {
+		int opt = 1;
+		setsockopt(sock, IPPROTO_TCP, TCP_NODELAY, &opt, sizeof(opt));
+	}
+	return res;
+}
+
 #if __APPLE__
 
+/* OS X does not have LD_PRELOAD but has DYLD_INSERT_LIBRARIES.  The right
+ * environment variable is set by sshfs.c when attempting to load the
+ * sshnodelay workaround.
+ *
+ * However, things are not that simple: DYLD_INSERT_LIBRARIES does not
+ * behave exactly like LD_PRELOAD.  Instead, the dyld dynamic linker will
+ * look for __DATA __interpose sections on the libraries given via the
+ * DYLD_INSERT_LIBRARIES variable.  The contents of this section are pairs
+ * of replacement functions and functions to be replaced, respectively.
+ * Prepare such section here. */
+
 int custom_connect(int sock, const struct sockaddr *addr, socklen_t addrlen);
 
 typedef struct interpose_s {
@@ -21,26 +45,15 @@ static const interpose_t interposers[] \
 
 int custom_connect(int sock, const struct sockaddr *addr, socklen_t addrlen)
 {
-	int res = connect(sock, addr, addrlen);
-	if (!res && addr->sa_family == AF_INET) {
-		int opt = 1;
-		setsockopt(sock, IPPROTO_TCP, TCP_NODELAY, &opt, sizeof(opt));
-	}
-	return res;
+	return nodelay_connect(connect, sock, addr, addrlen);
 }
 
 #else /* !__APPLE__ */
 
 int connect(int sock, const struct sockaddr *addr, socklen_t addrlen)
 {
-	int (*next_connect)(int, const struct sockaddr *, socklen_t) =
-		dlsym(RTLD_NEXT, "connect");
-	int res = next_connect(sock, addr, addrlen);
-	if (!res && addr->sa_family == AF_INET) {
-		int opt = 1;
-		setsockopt(sock, IPPROTO_TCP, TCP_NODELAY, &opt, sizeof(opt));
-	}
-	return res;
+	return nodelay_connect(dlsym(RTLD_NEXT, "connect"),
+	                       sock, addr, addrlen);
 }
 
 #endif /* !__APPLE__ */