Revert "Remove env config fetcher code"

This reverts commit 6324647bcc.

Reason for revert: This change negatively impacts users running builds on gLinux laptops. Reverting to limit impact and reland with appropriate fix.

Bug: b/284985772
Change-Id: I04a3d0107bb3ea680ba6641240620736a2ef5f17

ui/build/config.go

@@ -15,6 +15,7 @@
 package build
 
 import (
+	"context"
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
@@ -38,6 +39,9 @@ import (
 const (
 	envConfigDir = "vendor/google/tools/soong_config"
 	jsonSuffix   = "json"
+
+	configFetcher         = "vendor/google/tools/soong/expconfigfetcher"
+	envConfigFetchTimeout = 10 * time.Second
 )
 
 var (
@@ -166,6 +170,87 @@ func checkTopDir(ctx Context) {
 	}
 }
 
+// fetchEnvConfig optionally fetches a configuration file that can then subsequently be
+// loaded into Soong environment to control certain aspects of build behavior (e.g., enabling RBE).
+// If a configuration file already exists on disk, the fetch is run in the background
+// so as to NOT block the rest of the build execution.
+func fetchEnvConfig(ctx Context, config *configImpl, envConfigName string) error {
+	configName := envConfigName + "." + jsonSuffix
+	expConfigFetcher := &smpb.ExpConfigFetcher{Filename: &configName}
+	defer func() {
+		ctx.Metrics.ExpConfigFetcher(expConfigFetcher)
+	}()
+	if !config.GoogleProdCredsExist() {
+		status := smpb.ExpConfigFetcher_MISSING_GCERT
+		expConfigFetcher.Status = &status
+		return nil
+	}
+
+	s, err := os.Stat(configFetcher)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil
+		}
+		return err
+	}
+	if s.Mode()&0111 == 0 {
+		status := smpb.ExpConfigFetcher_ERROR
+		expConfigFetcher.Status = &status
+		return fmt.Errorf("configuration fetcher binary %v is not executable: %v", configFetcher, s.Mode())
+	}
+
+	configExists := false
+	outConfigFilePath := filepath.Join(config.OutDir(), configName)
+	if _, err := os.Stat(outConfigFilePath); err == nil {
+		configExists = true
+	}
+
+	tCtx, cancel := context.WithTimeout(ctx, envConfigFetchTimeout)
+	fetchStart := time.Now()
+	cmd := exec.CommandContext(tCtx, configFetcher, "-output_config_dir", config.OutDir(),
+		"-output_config_name", configName)
+	if err := cmd.Start(); err != nil {
+		status := smpb.ExpConfigFetcher_ERROR
+		expConfigFetcher.Status = &status
+		return err
+	}
+
+	fetchCfg := func() error {
+		if err := cmd.Wait(); err != nil {
+			status := smpb.ExpConfigFetcher_ERROR
+			expConfigFetcher.Status = &status
+			return err
+		}
+		fetchEnd := time.Now()
+		expConfigFetcher.Micros = proto.Uint64(uint64(fetchEnd.Sub(fetchStart).Microseconds()))
+		expConfigFetcher.Filename = proto.String(outConfigFilePath)
+
+		if _, err := os.Stat(outConfigFilePath); err != nil {
+			status := smpb.ExpConfigFetcher_NO_CONFIG
+			expConfigFetcher.Status = &status
+			return err
+		}
+		status := smpb.ExpConfigFetcher_CONFIG
+		expConfigFetcher.Status = &status
+		return nil
+	}
+
+	// If a config file does not exist, wait for the config file to be fetched. Otherwise
+	// fetch the config file in the background and return immediately.
+	if !configExists {
+		defer cancel()
+		return fetchCfg()
+	}
+
+	go func() {
+		defer cancel()
+		if err := fetchCfg(); err != nil {
+			ctx.Verbosef("Failed to fetch config file %v: %v\n", configName, err)
+		}
+	}()
+	return nil
+}
+
 func loadEnvConfig(ctx Context, config *configImpl, bc string) error {
 	if bc == "" {
 		return nil
@@ -265,6 +350,9 @@ func NewConfig(ctx Context, args ...string) Config {
 	bc := os.Getenv("ANDROID_BUILD_ENVIRONMENT_CONFIG")
 
 	if bc != "" {
+		if err := fetchEnvConfig(ctx, ret, bc); err != nil {
+			ctx.Verbosef("Failed to fetch config file: %v\n", err)
+		}
 		if err := loadEnvConfig(ctx, ret, bc); err != nil {
 			ctx.Fatalln("Failed to parse env config files: %v", err)
 		}