gopls/internal/lsp/cache: actually preload files in parallel

snapshot.ReadFile holds the snapshot lock, so calling it concurrently
was pointless. Instead, add a new preloadFiles helper that delegates to
to the underlying FileSource.

For golang/go#57987

Change-Id: If3259ca45260b8a24542d40f7558b5d6bf5018d1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/477975
gopls-CI: kokoro <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

gopls/internal/lsp/cache/load.go

@@ -12,7 +12,6 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
-	"sync"
 	"sync/atomic"
 	"time"
 
@@ -255,7 +254,7 @@ func (s *snapshot) load(ctx context.Context, allowNetwork bool, scopes ...loadSc
 	s.dumpWorkspace("load")
 	s.mu.Unlock()
 
-	// Opt: pre-fetch loaded files in parallel.
+	// Opt: preLoad files in parallel.
 	//
 	// Requesting files in batch optimizes the underlying filesystem reads.
 	// However, this is also currently necessary for correctness: populating all
@@ -266,16 +265,7 @@ func (s *snapshot) load(ctx context.Context, allowNetwork bool, scopes ...loadSc
 	// TODO(rfindley, golang/go#57558): determine the set of directories based on
 	// loaded packages, so that reading files here is not necessary for
 	// correctness.
-	var wg sync.WaitGroup
-	for _, uri := range files {
-		uri := uri
-		wg.Add(1)
-		go func() {
-			s.ReadFile(ctx, uri) // ignore result
-			wg.Done()
-		}()
-	}
-	wg.Wait()
+	s.preloadFiles(ctx, files)
 
 	if len(moduleErrs) > 0 {
 		return &moduleErrorMap{moduleErrs}

gopls/internal/lsp/cache/snapshot.go

@@ -1240,6 +1240,42 @@ func (s *snapshot) ReadFile(ctx context.Context, uri span.URI) (source.FileHandl
 	return lockedSnapshot{s}.ReadFile(ctx, uri)
 }
 
+// preloadFiles delegates to the view FileSource to read the requested uris in
+// parallel, without holding the snapshot lock.
+func (s *snapshot) preloadFiles(ctx context.Context, uris []span.URI) {
+	files := make([]source.FileHandle, len(uris))
+	var wg sync.WaitGroup
+	iolimit := make(chan struct{}, 20) // I/O concurrency limiting semaphore
+	for i, uri := range uris {
+		wg.Add(1)
+		iolimit <- struct{}{}
+		go func(i int, uri span.URI) {
+			defer wg.Done()
+			fh, err := s.view.fs.ReadFile(ctx, uri)
+			<-iolimit
+			if err != nil && ctx.Err() == nil {
+				event.Error(ctx, fmt.Sprintf("reading %s", uri), err)
+				return
+			}
+			files[i] = fh
+		}(i, uri)
+	}
+	wg.Wait()
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	for i, fh := range files {
+		if fh == nil {
+			continue // error logged above
+		}
+		uri := uris[i]
+		if _, ok := s.files.Get(uri); !ok {
+			s.files.Set(uri, fh)
+		}
+	}
+}
+
 // A lockedSnapshot implements the source.FileSource interface while holding
 // the lock for the wrapped snapshot.
 type lockedSnapshot struct{ wrapped *snapshot }