Leave control back to gen_server during supervisor's restart loop

When an attempt to restart a child failed, supervisor would earlier
keep the execution flow and try to restart the child over and over
again until it either succeeded or the restart frequency limit was
reached. If none of these happened, supervisor would hang forever in
this loop.

This commit adds a timer of 0 ms where the control is left back to the
gen_server which implements the supervisor. This way any incoming
request to the supervisor will be handled - which could help breaking
the infinite loop - e.g. shutdown request for the supervisor or for
the problematic child.

This introduces some incompatibilities in stdlib due to new return
values from supervisor:
       * restart_child/2 can now return {error,restarting}
       * delete_child/2 can now return {error,restarting}
       * which_children/1 returns a list of {Id,Child,Type,Mods},
         where Child, in addition to the old pid() or 'undefined',
         now also can be 'restarting'.

1 files changed, 7 insertions, 5 deletions

diff --git a/lib/stdlib/doc/src/supervisor.xml b/lib/stdlib/doc/src/supervisor.xml
index d1e62230bc..35f4f82264 100644
--- a/lib/stdlib/doc/src/supervisor.xml
+++ b/lib/stdlib/doc/src/supervisor.xml
@@ -399,10 +399,11 @@ child_spec() = {Id,StartFunc,Restart,Shutdown,Type,Modules}
           <c>SupRef</c>.</p>
         <p>If successful, the function returns <c>ok</c>. If the child
           specification identified by <c><anno>Id</anno></c> exists but
-          the corresponding child process is running, the function
-          returns <c>{error,running}</c>. If the child specification
-          identified by <c><anno>Id</anno></c> does not exist, the function returns
-          <c>{error,not_found}</c>.</p>
+          the corresponding child process is running or about to be restarted,
+	  the function returns <c>{error,running}</c> or
+	  <c>{error,restarting}</c> respectively. If the child specification
+          identified by <c><anno>Id</anno></c> does not exist, the function
+	  returns <c>{error,not_found}</c>.</p>
       </desc>
     </func>
     <func>
@@ -462,7 +463,8 @@ child_spec() = {Id,StartFunc,Restart,Shutdown,Type,Modules}
           </item>
           <item>
             <p><c><anno>Child</anno></c> - the pid of the corresponding child
-              process, or <c>undefined</c> if there is no such process.</p>
+              process, the atom <c>restarting</c> if the process is about to be
+	      restarted or <c>undefined</c> if there is no such process.</p>
           </item>
           <item>
             <p><c><anno>Type</anno></c> - as defined in the child specification.</p>