Fix possible synchronization issue in TaskThunk.

When cancel is called, it is executed in the caller thread, which is
different from the thread run() is executing on. Only one thread may
use the context at a time, so a big lock is needed around the two
methods.

Change-Id: I41c203761cef5ca84914c3ace8e33b704212cdc2

gerrit-sshd/src/main/java/com/google/gerrit/sshd/BaseCommand.java

diff --git a/gerrit-sshd/src/main/java/com/google/gerrit/sshd/BaseCommand.java b/gerrit-sshd/src/main/java/com/google/gerrit/sshd/BaseCommand.java
index 0327fb8..6e0d60a 100644
--- a/gerrit-sshd/src/main/java/com/google/gerrit/sshd/BaseCommand.java
+++ b/gerrit-sshd/src/main/java/com/google/gerrit/sshd/BaseCommand.java
@@ -379,55 +379,59 @@
 
     @Override
     public void cancel() {
-      final Context old = SshScope.set(context);
-      try {
-        onExit(STATUS_CANCEL);
-      } finally {
-        SshScope.set(old);
+      synchronized (this) {
+        final Context old = SshScope.set(context);
+        try {
+          onExit(STATUS_CANCEL);
+        } finally {
+          SshScope.set(old);
+        }
       }
     }
 
     @Override
     public void run() {
-      final Thread thisThread = Thread.currentThread();
-      final String thisName = thisThread.getName();
-      int rc = 0;
-      final Context old = SshScope.set(context);
-      try {
-        context.started = System.currentTimeMillis();
-        thisThread.setName("SSH " + taskName);
-
-        if (thunk instanceof ProjectCommandRunnable) {
-          ((ProjectCommandRunnable) thunk).executeParseCommand();
-          projectName = ((ProjectCommandRunnable) thunk).getProjectName();
-        }
-
+      synchronized (this) {
+        final Thread thisThread = Thread.currentThread();
+        final String thisName = thisThread.getName();
+        int rc = 0;
+        final Context old = SshScope.set(context);
         try {
-          thunk.run();
-        } catch (NoSuchProjectException e) {
-          throw new UnloggedFailure(1, e.getMessage() + " no such project");
-        } catch (NoSuchChangeException e) {
-          throw new UnloggedFailure(1, e.getMessage() + " no such change");
-        }
+          context.started = System.currentTimeMillis();
+          thisThread.setName("SSH " + taskName);
 
-        out.flush();
-        err.flush();
-      } catch (Throwable e) {
-        try {
+          if (thunk instanceof ProjectCommandRunnable) {
+            ((ProjectCommandRunnable) thunk).executeParseCommand();
+            projectName = ((ProjectCommandRunnable) thunk).getProjectName();
+          }
+
+          try {
+            thunk.run();
+          } catch (NoSuchProjectException e) {
+            throw new UnloggedFailure(1, e.getMessage() + " no such project");
+          } catch (NoSuchChangeException e) {
+            throw new UnloggedFailure(1, e.getMessage() + " no such change");
+          }
+
           out.flush();
-        } catch (Throwable e2) {
-        }
-        try {
           err.flush();
-        } catch (Throwable e2) {
-        }
-        rc = handleError(e);
-      } finally {
-        try {
-          onExit(rc);
+        } catch (Throwable e) {
+          try {
+            out.flush();
+          } catch (Throwable e2) {
+          }
+          try {
+            err.flush();
+          } catch (Throwable e2) {
+          }
+          rc = handleError(e);
         } finally {
-          SshScope.set(old);
-          thisThread.setName(thisName);
+          try {
+            onExit(rc);
+          } finally {
+            SshScope.set(old);
+            thisThread.setName(thisName);
+          }
         }
       }
     }