fs: fix and document exec deadlock

Change-Id: Id00000007e5deb860c535755f71cf3299721ebc6

Author: hanwen

fs/api.go

@@ -207,6 +207,11 @@
 // by calling "fusermount" with an inherited file descriptor, but the
 // same problem may occur for other file descriptors.
 //
+// 1c. If the executable is on the FUSE mount. In this case, the child
+// calls exec, which reads the file to execute, which triggers an OPEN
+// opcode. This can be worked around by invoking the subprocess
+// through a wrapper, eg `bash -c file/on/fuse-mount`.
+//
 // 2. The Go runtime uses the epoll system call to understand which
 // goroutines can respond to I/O.  The runtime assumes that epoll does
 // not block, but if files are on a FUSE filesystem, the kernel will

fs/simple_test.go

@@ -173,7 +173,7 @@ func TestExec(t *testing.T) {
 	tc := newTestCase(t, &testOptions{attrCache: true, entryCache: true})
 	tc.writeOrig("test.sh", "#!/bin/sh\ntrue\n", 0755)
 	fn := tc.mntDir + "/test.sh"
-	if err := exec.Command(fn).Run(); err != nil {
+	if err := exec.Command("bash", "-c", fn).Run(); err != nil {
 		t.Fatalf("Run: %v", err)
 	}
 }