fix #4231: memory leak inside bundler

Author: evanw

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+* Fix a memory leak when `cancel()` is used on a build context ([#4231](https://github.com/evanw/esbuild/issues/4231))
+
+    Calling `rebuild()` followed by `cancel()` in rapid succession could previously leak memory. The bundler uses a producer/consumer model internally, and the resource leak was caused by the consumer being termianted while there were still remaining unreceived results from a producer. To avoid the leak, the consumer now waits for all producers to finish before terminating.
+
 * Consider negated bigints to have no side effects
 
     While esbuild currently considers `1`, `-1`, and `1n` to all have no side effects, it didn't previously consider `-1n` to have no side effects. This is because esbuild does constant folding with numbers but not bigints. However, it meant that unused negative bigint constants were not tree-shaken. With this release, esbuild will now consider these expressions to also be side-effect free:

internal/bundler/bundler.go

@@ -1357,6 +1357,17 @@ func ScanBundle(
 		}
 	}()
 
+	// Always consume all unused results. Failing to do so could cause a memory
+	// leak if a build is cancelled. We can't cancel the producers by closing the
+	// channel because doing so in Go causes a panic (which is arguably a design
+	// bug with Go).
+	defer func() {
+		for s.remaining > 0 {
+			<-s.resultChannel
+			s.remaining--
+		}
+	}()
+
 	// Wait for all "onStart" plugins here before continuing. People sometimes run
 	// setup code in "onStart" that "onLoad" expects to be able to use without
 	// "onLoad" needing to block on the completion of their "onStart" callback.