diff --git a/src/cmd/go/internal/cache/cache.go b/src/cmd/go/internal/cache/cache.go
index c9acd8782d..1bef1db08c 100644
--- a/src/cmd/go/internal/cache/cache.go
+++ b/src/cmd/go/internal/cache/cache.go
@@ -296,19 +296,19 @@ func GetBytes(c Cache, id ActionID) ([]byte, Entry, error) {
 // GetMmap looks up the action ID in the cache and returns
 // the corresponding output bytes.
 // GetMmap should only be used for data that can be expected to fit in memory.
-func GetMmap(c Cache, id ActionID) ([]byte, Entry, error) {
+func GetMmap(c Cache, id ActionID) ([]byte, Entry, bool, error) {
 	entry, err := c.Get(id)
 	if err != nil {
-		return nil, entry, err
+		return nil, entry, false, err
 	}
-	md, err := mmap.Mmap(c.OutputFile(entry.OutputID))
+	md, opened, err := mmap.Mmap(c.OutputFile(entry.OutputID))
 	if err != nil {
-		return nil, Entry{}, err
+		return nil, Entry{}, opened, err
 	}
 	if int64(len(md.Data)) != entry.Size {
-		return nil, Entry{}, &entryNotFoundError{Err: errors.New("file incomplete")}
+		return nil, Entry{}, true, &entryNotFoundError{Err: errors.New("file incomplete")}
 	}
-	return md.Data, entry, nil
+	return md.Data, entry, true, nil
 }
 
 // OutputFile returns the name of the cache file storing output with the given OutputID.
diff --git a/src/cmd/go/internal/mmap/mmap.go b/src/cmd/go/internal/mmap/mmap.go
index fcbd3e08c1..fd374df82e 100644
--- a/src/cmd/go/internal/mmap/mmap.go
+++ b/src/cmd/go/internal/mmap/mmap.go
@@ -22,10 +22,11 @@ type Data struct {
 }
 
 // Mmap maps the given file into memory.
-func Mmap(file string) (Data, error) {
+func Mmap(file string) (Data, bool, error) {
 	f, err := os.Open(file)
 	if err != nil {
-		return Data{}, err
+		return Data{}, false, err
 	}
-	return mmapFile(f)
+	data, err := mmapFile(f)
+	return data, true, err
 }
diff --git a/src/cmd/go/internal/modindex/read.go b/src/cmd/go/internal/modindex/read.go
index c4102409b4..4c1fbd8359 100644
--- a/src/cmd/go/internal/modindex/read.go
+++ b/src/cmd/go/internal/modindex/read.go
@@ -183,16 +183,21 @@ func openIndexModule(modroot string, ismodcache bool) (*Module, error) {
 		if err != nil {
 			return nil, err
 		}
-		data, _, err := cache.GetMmap(cache.Default(), id)
+		data, _, opened, err := cache.GetMmap(cache.Default(), id)
 		if err != nil {
 			// Couldn't read from modindex. Assume we couldn't read from
 			// the index because the module hasn't been indexed yet.
+			// But double check on Windows that we haven't opened the file yet,
+			// because once mmap opens the file, we can't close it, and
+			// Windows won't let us open an already opened file.
 			data, err = indexModule(modroot)
 			if err != nil {
 				return nil, err
 			}
-			if err = cache.PutBytes(cache.Default(), id, data); err != nil {
-				return nil, err
+			if runtime.GOOS != "windows" || !opened {
+				if err = cache.PutBytes(cache.Default(), id, data); err != nil {
+					return nil, err
+				}
 			}
 		}
 		mi, err := fromBytes(modroot, data)
@@ -212,13 +217,18 @@ func openIndexPackage(modroot, pkgdir string) (*IndexPackage, error) {
 		if err != nil {
 			return nil, err
 		}
-		data, _, err := cache.GetMmap(cache.Default(), id)
+		data, _, opened, err := cache.GetMmap(cache.Default(), id)
 		if err != nil {
 			// Couldn't read from index. Assume we couldn't read from
 			// the index because the package hasn't been indexed yet.
+			// But double check on Windows that we haven't opened the file yet,
+			// because once mmap opens the file, we can't close it, and
+			// Windows won't let us open an already opened file.
 			data = indexPackage(modroot, pkgdir)
-			if err = cache.PutBytes(cache.Default(), id, data); err != nil {
-				return nil, err
+			if runtime.GOOS != "windows" || !opened {
+				if err = cache.PutBytes(cache.Default(), id, data); err != nil {
+					return nil, err
+				}
 			}
 		}
 		pkg, err := packageFromBytes(modroot, data)