Explicitly declare a channel nonblocking before throwing EAGAIN on write.

generic/tclIO.c

@@ -2118,7 +2118,7 @@ Tcl_GetChannelHandle(
     chanPtr = ((Channel *) chan)->state->bottomChanPtr;
     if (!chanPtr->typePtr->getHandleProc) {
         Tcl_SetChannelError(chan, Tcl_ObjPrintf(
-                "channel \"%s\" does not support OS handles",
+                "{channel \"%s\" does not support OS handles}",
                 Tcl_GetChannelName(chan)));
 	return TCL_ERROR;
     }
@@ -2453,13 +2453,35 @@ FlushChannel(
 	     */
 
 	    if ((errorCode == EWOULDBLOCK) || (errorCode == EAGAIN)) {
-		/*
-		 * This used to check for CHANNEL_NONBLOCKING, and panic if
+
+                
+		/* PREVIOUS RATIONALE -- UNDER EVALUATION
+                 *
+                 * This used to check for CHANNEL_NONBLOCKING, and panic if
 		 * the channel was blocking. However, it appears that setting
 		 * stdin to -blocking 0 has some effect on the stdout when
 		 * it's a tty channel (dup'ed underneath)
 		 */
 
+                /* NEW RATIONALE -- UNDER EVALUATION
+                 *
+                 * We explicitly disallow EAGAIN/EWOULDBLOCK for blocking
+                 * channels, but throw an error instead of panicking, since
+                 * this may be a simple scripting error in a refelected
+                 * channel.
+                 */
+		
+                if (!GotFlag(statePtr, CHANNEL_NONBLOCKING)) {
+                    //Tcl_Panic("EAGAIN Illegally thrown by blocking channel");
+
+                    Tcl_SetChannelError((Tcl_Channel) chanPtr,
+                                        Tcl_ObjPrintf("{EAGAIN Illegally thrown by blocking channel \"%s\"}",
+                                                      Tcl_GetChannelName((Tcl_Channel)chanPtr)));
+                    errorCode=EINVAL;
+                    DiscardOutputQueued(statePtr);
+                    goto done;
+                }
+
 		if (!GotFlag(statePtr, BG_FLUSH_SCHEDULED)) {
 		    SetFlag(statePtr, BG_FLUSH_SCHEDULED);
 		    UpdateInterest(chanPtr);
@@ -2524,7 +2546,6 @@ FlushChannel(
 	     * When we get an error we throw away all the output currently
 	     * queued.
 	     */
-
 	    DiscardOutputQueued(statePtr);
 	    continue;
 	} else {

tests/ioCmd.test

@@ -1214,6 +1214,7 @@ test iocmd-24.15 {chan write, EAGAIN means that writing is not allowed at this t
 	error EAGAIN
     }
     set c [chan create {r w} foo]
+    fconfigure $c -blocking 0
 } -body {
     note [puts -nonewline $c ABC ; flush $c]
     set res
@@ -2566,6 +2567,7 @@ test iocmd.tf-24.15 {chan write, EAGAIN means that writing is not allowed at thi
 	error EAGAIN
     }
     set c [chan create {r w} foo]
+    fconfigure $c -blocking 0
 } -body {
     notes [inthread $c {
 	note [puts -nonewline $c ABC ; flush $c]
@@ -2575,6 +2577,7 @@ test iocmd.tf-24.15 {chan write, EAGAIN means that writing is not allowed at thi
     set res
 } -cleanup {
     proc foo {args} {onfinal; set ::done-24.15 1; return 3}
+    after 1000 {set ::done-24.15 2}
     vwait done-24.15
     rename foo {}
     unset res
@@ -2592,6 +2595,7 @@ test iocmd.tf-24.16 {chan write, note the background flush setup by close due to
 	error EAGAIN
     }
     set c [chan create {r w} foo]
+    fconfigure $c -blocking 0
 } -body {
     notes [inthread $c {
 	note [puts -nonewline $c ABC ; flush $c]
@@ -2603,10 +2607,12 @@ test iocmd.tf-24.16 {chan write, note the background flush setup by close due to
     proc foo {args} { onfinal; note BG ; track ; set ::endbody-24.16 1}
     # Flush (sic!) the event-queue to capture the write from a
     # BG-flush.
+    after 1000 {set ::endbody-24.16 2}
     vwait endbody-24.16
     set res
 } -cleanup {
     proc foo {args} {onfinal; set ::done-24.16 1; return 3}
+    after 1000 {set ::done-24.16 2}
     vwait done-24.16
     rename foo {}
     unset res