runtime: Add memprofrate value to GODEBUG

Add memprofrate as a value recognized in GODEBUG.  The
value provided is used as the new setting for
runtime.MemProfileRate, allowing the user to
adjust memory profiling.

Change-Id: If129a247683263b11e2dd42473cf9b31280543d5
Reviewed-on: https://go-review.googlesource.com/3450Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/extern.go

@@ -45,6 +45,10 @@ a comma-separated list of name=val pairs. Supported names are:
 	This should only be used as a temporary workaround to diagnose buggy code.
 	The real fix is to not store integers in pointer-typed locations.
 
+	memprofrate: setting memprofrate=X will update the value of runtime.MemProfileRate.
+	When set to 0 memory profiling is disabled.  Refer to the description of
+	MemProfileRate for the default value.
+
 	scheddetail: setting schedtrace=X and scheddetail=1 causes the scheduler to emit
 	detailed multiline info every X milliseconds, describing state of the scheduler,
 	processors, threads and goroutines.

src/runtime/runtime1.go

@@ -308,7 +308,10 @@ type dbgVar struct {
 
 // TODO(rsc): Make GC respect debug.invalidptr.
 
-// Holds variables parsed from GODEBUG env var.
+// Holds variables parsed from GODEBUG env var,
+// except for "memprofrate" since there is an
+// existing int var for that value, which may
+// already have an initial value.
 var debug struct {
 	allocfreetrace int32
 	efence         int32
@@ -352,9 +355,17 @@ func parsedebugvars() {
 			continue
 		}
 		key, value := field[:i], field[i+1:]
-		for _, v := range dbgvars {
-			if v.name == key {
-				*v.value = int32(atoi(value))
+
+		// Update MemProfileRate directly here since it
+		// int, not int32, and should only be updated
+		// if specified in GODEBUG.
+		if key == "memprofrate" {
+			MemProfileRate = atoi(value)
+		} else {
+			for _, v := range dbgvars {
+				if v.name == key {
+					*v.value = int32(atoi(value))
+				}
 			}
 		}
 	}