Introduce official environment variable for goroutine limiting.

We've had to tell people how to do this, so we should have a name for it that doesn't refer to a defunct company.

change-notes/1.24/extractor-go.md

@@ -4,6 +4,9 @@
 
 ## Changes to code extraction
 
+* In resource-constrained environments, the environment variable `CODEQL_EXTRACTOR_GO_MAX_GOROUTINES` can be used to limit the
+  number of parallel goroutines started by the extractor, which reduces CPU and memory requirements. The default value for this
+  variable is 32.
 * The autobuilder now runs Makefiles or custom build scripts present in the codebase to install dependencies. The build command
   to invoke can be configured via `lgtm.yml`, or by setting the environment variable `CODEQL_EXTRACTOR_GO_BUILD_COMMAND`.
 * The autobuilder now attempts to automatically detect when dependencies have been vendored and use `-mod=vendor` appropriately.

extractor/cli/go-autobuilder/go-autobuilder.go

@@ -11,6 +11,8 @@ import (
 	"regexp"
 	"runtime"
 	"strings"
+
+	"github.com/github/codeql-go/extractor/util"
 )
 
 func usage() {
@@ -32,6 +34,11 @@ to 'false' disables the GOPATH set-up, CODEQL_EXTRACTOR_GO_BUILD_COMMAND (or alt
 LGTM_INDEX_BUILD_COMMAND), can be set to a newline-separated list of commands to run in order to
 install dependencies, and LGTM_INDEX_IMPORT_PATH can be used to override the package import path,
 which is otherwise inferred from the SEMMLE_REPO_URL environment variable.
+
+In resource-constrained environments, the environment variable CODEQL_EXTRACTOR_GO_MAX_GOROUTINES
+(or its legacy alias SEMMLE_MAX_GOROUTINES) can be used to limit the number of parallel goroutines
+started by the extractor, which reduces CPU and memory requirements. The default value for this
+variable is 32.
 `,
 		os.Args[0])
 	fmt.Fprintf(os.Stderr, "Usage:\n\n  %s\n", os.Args[0])
@@ -276,10 +283,7 @@ func main() {
 	}
 
 	// check whether an explicit dependency installation command was provided
-	inst := os.Getenv("CODEQL_EXTRACTOR_GO_BUILD_COMMAND")
-	if inst == "" {
-		inst = os.Getenv("LGTM_INDEX_BUILD_COMMAND")
-	}
+	inst := util.Getenv("CODEQL_EXTRACTOR_GO_BUILD_COMMAND", "LGTM_INDEX_BUILD_COMMAND")
 	var install *exec.Cmd
 	if inst == "" {
 		// if there is a build file, run the corresponding build tool

extractor/extractor.go

@@ -20,6 +20,7 @@ import (
 	"github.com/github/codeql-go/extractor/dbscheme"
 	"github.com/github/codeql-go/extractor/srcarchive"
 	"github.com/github/codeql-go/extractor/trap"
+	"github.com/github/codeql-go/extractor/util"
 	"golang.org/x/tools/go/packages"
 )
 
@@ -89,7 +90,10 @@ func ExtractWithFlags(buildFlags []string, patterns []string) error {
 	// available since Go 1.5, but is subject to change
 
 	var maxgoroutines int
-	if maxgoroutines, err = strconv.Atoi(os.Getenv("SEMMLE_MAX_GOROUTINES")); err != nil {
+	if maxgoroutines, err = strconv.Atoi(util.Getenv(
+		"CODEQL_EXTRACTOR_GO_MAX_GOROUTINES",
+		"SEMMLE_MAX_GOROUTINES",
+	)); err != nil {
 		maxgoroutines = 32
 	} else {
 		log.Printf("Max goroutines set to %d", maxgoroutines)

extractor/util/util.go

@@ -0,0 +1,20 @@
+package util
+
+import "os"
+
+// Getenv retrieves the value of the environment variable named by the key.
+// If that variable is not present, it iterates over the given aliases until
+// it finds one that is. If none are present, the empty string is returned.
+func Getenv(key string, aliases ...string) string {
+	val := os.Getenv(key)
+	if val != "" {
+		return val
+	}
+	for _, alias := range aliases {
+		val = os.Getenv(alias)
+		if val != "" {
+			return val
+		}
+	}
+	return ""
+}