cmd/go: disable SSH connection pooling to avoid git hang

Fixes #13453.
Fixes #16104.

Change-Id: I4e94f606df786af8143f8649c9afde570f346301
Reviewed-on: https://go-review.googlesource.com/31353Reviewed-by: Quentin Smith <quentin@golang.org>

src/cmd/go/get.go

@@ -98,6 +98,23 @@ func runGet(cmd *Command, args []string) {
 		os.Setenv("GIT_TERMINAL_PROMPT", "0")
 	}
 
+	// Disable any ssh connection pooling by Git.
+	// If a Git subprocess forks a child into the background to cache a new connection,
+	// that child keeps stdout/stderr open. After the Git subprocess exits,
+	// os /exec expects to be able to read from the stdout/stderr pipe
+	// until EOF to get all the data that the Git subprocess wrote before exiting.
+	// The EOF doesn't come until the child exits too, because the child
+	// is holding the write end of the pipe.
+	// This is unfortunate, but it has come up at least twice
+	// (see golang.org/issue/13453 and golang.org/issue/16104)
+	// and confuses users when it does.
+	// If the user has explicitly set GIT_SSH or GIT_SSH_COMMAND,
+	// assume they know what they are doing and don't step on it.
+	// But default to turning off ControlMaster.
+	if os.Getenv("GIT_SSH") == "" && os.Getenv("GIT_SSH_COMMAND") == "" {
+		os.Setenv("GIT_SSH_COMMAND", "ssh -o ControlMaster=no")
+	}
+
 	// Phase 1.  Download/update.
 	var stk importStack
 	mode := 0