git-gui: make _shellpath usable on startup

Since commit d5257fb (git-gui: handle textconv filter on
Windows and in development, 2010-08-07), git-gui will search for a
usable shell if _shellpath is not configured, and on Windows may
resort to using auto_execok to find 'sh'. While this was intended for
development use, checks are insufficient to assure a proper
configuration when deployed where _shellpath is always set, but might
not give a usable shell.

Let's make this more robust by only searching if _shellpath was not
defined, and then using only our restricted search functions.
Furthermore, we should convert to a Windows path on Windows.  Always
check for a valid shell on startup, meaning an absolute path to an
executable, aborting if these conditions are not met.

Signed-off-by: Mark Levedahl <[email protected]>
Signed-off-by: Johannes Sixt <[email protected]>

Signed-off-by: Taylor Blau <[email protected]>

Author: mark987

git-gui.sh

@@ -307,15 +307,37 @@ if {$_trace >= 0} {
 # branches).
 set _last_merged_branch {}
 
-proc shellpath {} {
-	global _shellpath env
-	if {[string match @@* $_shellpath]} {
-		if {[info exists env(SHELL)]} {
-			return $env(SHELL)
-		} else {
-			return /bin/sh
-		}
+# for testing, allow unconfigured _shellpath
+if {[string match @@* $_shellpath]} {
+	if {[info exists env(SHELL)]} {
+		set _shellpath $env(SHELL)
+	} else {
+		set _shellpath /bin/sh
 	}
+}
+
+if {[is_Windows]} {
+	set _shellpath [exec cygpath -m $_shellpath]
+}
+
+if {![file executable $_shellpath] || \
+	!([file pathtype $_shellpath] eq {absolute})} {
+	set errmsg "The defined shell ('$_shellpath') is not usable, \
+		it must be an absolute path to an executable."
+	puts stderr $errmsg
+
+	catch {wm withdraw .}
+	tk_messageBox \
+		-icon error \
+		-type ok \
+		-title "git-gui: configuration error" \
+		-message $errmsg
+	exit 1
+}
+
+
+proc shellpath {} {
+	global _shellpath
 	return $_shellpath
 }