Merge "Set GODEBUG=asyncpreemptoff=1 for soong_build."

bootstrap/bootstrap.go

@@ -124,10 +124,27 @@ var (
 			// better to not to touch that while Blueprint and Soong are separate
 			// NOTE: The spaces at EOL are important because otherwise Ninja would
 			// omit all spaces between the different options.
+
+			// GODEBUG=asyncpreemptoff=1 disables the preemption of goroutines. This
+			// is useful because the preemption happens by sending SIGURG to the OS
+			// thread hosting the goroutine in question and each signal results in
+			// work that needs to be done by Delve; it uses ptrace to debug the Go
+			// process and the tracer process must deal with every signal (it is not
+			// possible to selectively ignore SIGURG). This makes debugging slower,
+			// sometimes by an order of magnitude depending on luck.
+			//
+			// According to measurements, this does not result in any performance loss
+			// (or gain), but if it becomes necessary for some reason, it should be
+			// possible to set asyncpreemptoff=1 only if debugging is enabled by
+			// plumbing that information through PrimaryBuilderInvocation just like
+			// --delve_path and --delve_listen.
+			//
+			// The original reason for adding async preemption to Go is here:
+			// https://github.com/golang/proposal/blob/master/design/24543-non-cooperative-preemption.md
 			Command: `cd "$$(dirname "$builder")" && ` +
 				`BUILDER="$$PWD/$$(basename "$builder")" && ` +
 				`cd / && ` +
-				`env -i "$$BUILDER" ` +
+				`env -i GODEBUG=asyncpreemptoff=1 "$$BUILDER" ` +
 				`    --top "$$TOP" ` +
 				`    --soong_out "$soongOutDir" ` +
 				`    --out "$outDir" ` +