Fix null return from FS.readPipe when command fails to launch

When a command invoked from readPipe fails to launch (i.e. the exec call
fails due to a missing command executable), Process.start() throws,
which gets caught by the generic IOException handler, resulting in a
null return. This change detects this case and rethrows a
CommandFailedException instead.

Additionally, this change uses /bin/sh instead of bash for its posix
command failure test, to accomodate building in environments where bash
is unavailable.

Change-Id: Ifae51e457e5718be610c0a0914b18fe35ea7b008
Signed-off-by: Bryan Donlan <bdonlan@gmail.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/util/FSTest.java

@@ -172,9 +172,18 @@ public class FSTest {
 		FS fs = FS.DETECTED.newInstance();
 		assumeTrue(fs instanceof FS_POSIX);
 
-		String r = FS.readPipe(fs.userHome(),
-				new String[] { "bash", "--login", "-c", "foobar" },
+		FS.readPipe(fs.userHome(),
+				new String[] { "/bin/sh", "-c", "exit 1" },
 				Charset.defaultCharset().name());
-		System.out.println(r);
+	}
+
+	@Test(expected = CommandFailedException.class)
+	public void testReadPipeCommandStartFailure()
+			throws CommandFailedException {
+		FS fs = FS.DETECTED.newInstance();
+
+		FS.readPipe(fs.userHome(),
+				  new String[] { "this-command-does-not-exist" },
+				  Charset.defaultCharset().name());
 	}
 }

org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java

@@ -497,7 +497,13 @@ public abstract class FS {
 			if (env != null) {
 				pb.environment().putAll(env);
 			}
-			Process p = pb.start();
+			Process p;
+			try {
+				p = pb.start();
+			} catch (IOException e) {
+				// Process failed to start
+				throw new CommandFailedException(-1, e.getMessage(), e);
+			}
 			p.getOutputStream().close();
 			GobblerThread gobbler = new GobblerThread(p, command, dir);
 			gobbler.start();