cmd/go: implement time-based trimming of build cache

Fixes #22642.

Change-Id: I2ed6305555a0cf753b9cdce061463b1749d5e53e
Reviewed-on: https://go-review.googlesource.com/81975
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

3 files changed, 215 insertions, 6 deletions

diff --git a/src/cmd/go/internal/cache/cache.go b/src/cmd/go/internal/cache/cache.go
index 311cd89f33..794d63d20b 100644
--- a/src/cmd/go/internal/cache/cache.go
+++ b/src/cmd/go/internal/cache/cache.go
@@ -178,9 +178,7 @@ func (c *Cache) get(id ActionID) (Entry, error) {
 
 	fmt.Fprintf(c.log, "%d get %x\n", c.now().Unix(), id)
 
-	// Best-effort attempt to update mtime on file,
-	// so that mtime reflects cache access time.
-	os.Chtimes(c.fileName(id, "a"), c.now(), c.now())
+	c.used(c.fileName(id, "a"))
 
 	return Entry{buf, size, time.Unix(0, tm)}, nil
 }
@@ -203,12 +201,95 @@ func (c *Cache) GetBytes(id ActionID) ([]byte, Entry, error) {
 // OutputFile returns the name of the cache file storing output with the given OutputID.
 func (c *Cache) OutputFile(out OutputID) string {
 	file := c.fileName(out, "d")
+	c.used(file)
+	return file
+}
 
-	// Best-effort attempt to update mtime on file,
-	// so that mtime reflects cache access time.
+// Time constants for cache expiration.
+//
+// We set the mtime on a cache file on each use, but at most one per mtimeInterval (1 hour),
+// to avoid causing many unnecessary inode updates. The mtimes therefore
+// roughly reflect "time of last use" but may in fact be older by at most an hour.
+//
+// We scan the cache for entries to delete at most once per trimInterval (1 day).
+//
+// When we do scan the cache, we delete entries that have not been used for
+// at least trimLimit (5 days). Statistics gathered from a month of usage by
+// Go developers found that essentially all reuse of cached entries happened
+// within 5 days of the previous reuse. See golang.org/issue/22990.
+const (
+	mtimeInterval = 1 * time.Hour
+	trimInterval  = 24 * time.Hour
+	trimLimit     = 5 * 24 * time.Hour
+)
+
+// used makes a best-effort attempt to update mtime on file,
+// so that mtime reflects cache access time.
+//
+// Because the reflection only needs to be approximate,
+// and to reduce the amount of disk activity caused by using
+// cache entries, used only updates the mtime if the current
+// mtime is more than an hour old. This heuristic eliminates
+// nearly all of the mtime updates that would otherwise happen,
+// while still keeping the mtimes useful for cache trimming.
+func (c *Cache) used(file string) {
+	info, err := os.Stat(file)
+	if err == nil && c.now().Sub(info.ModTime()) < mtimeInterval {
+		return
+	}
 	os.Chtimes(file, c.now(), c.now())
+}
 
-	return file
+// Trim removes old cache entries that are likely not to be reused.
+func (c *Cache) Trim() {
+	now := c.now()
+
+	// We maintain in dir/trim.txt the time of the last completed cache trim.
+	// If the cache has been trimmed recently enough, do nothing.
+	// This is the common case.
+	data, _ := ioutil.ReadFile(filepath.Join(c.dir, "trim.txt"))
+	t, err := strconv.ParseInt(strings.TrimSpace(string(data)), 10, 64)
+	if err == nil && now.Sub(time.Unix(t, 0)) < trimInterval {
+		return
+	}
+
+	// Trim each of the 256 subdirectories.
+	// We subtract an additional mtimeInterval
+	// to account for the imprecision of our "last used" mtimes.
+	cutoff := now.Add(-trimLimit - mtimeInterval)
+	for i := 0; i < 256; i++ {
+		subdir := filepath.Join(c.dir, fmt.Sprintf("%02x", i))
+		c.trimSubdir(subdir, cutoff)
+	}
+
+	ioutil.WriteFile(filepath.Join(c.dir, "trim.txt"), []byte(fmt.Sprintf("%d", now.Unix())), 0666)
+}
+
+// trimSubdir trims a single cache subdirectory.
+func (c *Cache) trimSubdir(subdir string, cutoff time.Time) {
+	// Read all directory entries from subdir before removing
+	// any files, in case removing files invalidates the file offset
+	// in the directory scan. Also, ignore error from f.Readdirnames,
+	// because we don't care about reporting the error and we still
+	// want to process any entries found before the error.
+	f, err := os.Open(subdir)
+	if err != nil {
+		return
+	}
+	names, _ := f.Readdirnames(-1)
+	f.Close()
+
+	for _, name := range names {
+		// Remove only cache entries (xxxx-a and xxxx-d).
+		if !strings.HasSuffix(name, "-a") && !strings.HasSuffix(name, "-d") {
+			continue
+		}
+		entry := filepath.Join(subdir, name)
+		info, err := os.Stat(entry)
+		if err == nil && info.ModTime().Before(cutoff) {
+			os.Remove(entry)
+		}
+	}
 }
 
 // putIndexEntry adds an entry to the cache recording that executing the action
@@ -239,6 +320,7 @@ func (c *Cache) putIndexEntry(id ActionID, out OutputID, size int64, allowVerify
 		os.Remove(file)
 		return err
 	}
+	os.Chtimes(file, c.now(), c.now()) // mainly for tests
 
 	fmt.Fprintf(c.log, "%d put %x %x %d\n", c.now().Unix(), id, out, size)
 	return nil
@@ -365,6 +447,7 @@ func (c *Cache) copyFile(file io.ReadSeeker, out OutputID, size int64) error {
 		os.Remove(name)
 		return err
 	}
+	os.Chtimes(name, c.now(), c.now()) // mainly for tests
 
 	return nil
 }
diff --git a/src/cmd/go/internal/cache/cache_test.go b/src/cmd/go/internal/cache/cache_test.go
index 84749a6965..d3dafccd13 100644
--- a/src/cmd/go/internal/cache/cache_test.go
+++ b/src/cmd/go/internal/cache/cache_test.go
@@ -5,7 +5,9 @@
 package cache
 
 import (
+	"bytes"
 	"encoding/binary"
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -196,3 +198,122 @@ func dummyID(x int) [HashSize]byte {
 	binary.LittleEndian.PutUint64(out[:], uint64(x))
 	return out
 }
+
+func TestCacheTrim(t *testing.T) {
+	dir, err := ioutil.TempDir("", "cachetest-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	c, err := Open(dir)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	const start = 1000000000
+	now := int64(start)
+	c.now = func() time.Time { return time.Unix(now, 0) }
+
+	checkTime := func(name string, mtime int64) {
+		t.Helper()
+		file := filepath.Join(c.dir, name[:2], name)
+		info, err := os.Stat(file)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if info.ModTime().Unix() != mtime {
+			t.Fatalf("%s mtime = %d, want %d", name, info.ModTime().Unix(), mtime)
+		}
+	}
+
+	id := ActionID(dummyID(1))
+	c.PutBytes(id, []byte("abc"))
+	entry, _ := c.Get(id)
+	c.PutBytes(ActionID(dummyID(2)), []byte("def"))
+	mtime := now
+	checkTime(fmt.Sprintf("%x-a", id), mtime)
+	checkTime(fmt.Sprintf("%x-d", entry.OutputID), mtime)
+
+	// Get should not change recent mtimes.
+	now = start + 10
+	c.Get(id)
+	checkTime(fmt.Sprintf("%x-a", id), mtime)
+	checkTime(fmt.Sprintf("%x-d", entry.OutputID), mtime)
+
+	// Get should change distant mtimes.
+	now = start + 5000
+	mtime2 := now
+	if _, err := c.Get(id); err != nil {
+		t.Fatal(err)
+	}
+	c.OutputFile(entry.OutputID)
+	checkTime(fmt.Sprintf("%x-a", id), mtime2)
+	checkTime(fmt.Sprintf("%x-d", entry.OutputID), mtime2)
+
+	// Trim should leave everything alone: it's all too new.
+	c.Trim()
+	if _, err := c.Get(id); err != nil {
+		t.Fatal(err)
+	}
+	c.OutputFile(entry.OutputID)
+	data, err := ioutil.ReadFile(filepath.Join(dir, "trim.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	checkTime(fmt.Sprintf("%x-a", dummyID(2)), start)
+
+	// Trim less than a day later should not do any work at all.
+	now = start + 80000
+	c.Trim()
+	if _, err := c.Get(id); err != nil {
+		t.Fatal(err)
+	}
+	c.OutputFile(entry.OutputID)
+	data2, err := ioutil.ReadFile(filepath.Join(dir, "trim.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(data, data2) {
+		t.Fatalf("second trim did work: %q -> %q", data, data2)
+	}
+
+	// Fast forward and do another trim just before the 5 day cutoff.
+	// Note that because of usedQuantum the cutoff is actually 5 days + 1 hour.
+	// We used c.Get(id) just now, so 5 days later it should still be kept.
+	// On the other hand almost a full day has gone by since we wrote dummyID(2)
+	// and we haven't looked at it since, so 5 days later it should be gone.
+	now += 5 * 86400
+	checkTime(fmt.Sprintf("%x-a", dummyID(2)), start)
+	c.Trim()
+	if _, err := c.Get(id); err != nil {
+		t.Fatal(err)
+	}
+	c.OutputFile(entry.OutputID)
+	mtime3 := now
+	if _, err := c.Get(dummyID(2)); err == nil { // haven't done a Get for this since original write above
+		t.Fatalf("Trim did not remove dummyID(2)")
+	}
+
+	// The c.Get(id) refreshed id's mtime again.
+	// Check that another 5 days later it is still not gone,
+	// but check by using checkTime, which doesn't bring mtime forward.
+	now += 5 * 86400
+	c.Trim()
+	checkTime(fmt.Sprintf("%x-a", id), mtime3)
+	checkTime(fmt.Sprintf("%x-d", entry.OutputID), mtime3)
+
+	// Half a day later Trim should still be a no-op, because there was a Trim recently.
+	// Even though the entry for id is now old enough to be trimmed,
+	// it gets a reprieve until the time comes for a new Trim scan.
+	now += 86400 / 2
+	c.Trim()
+	checkTime(fmt.Sprintf("%x-a", id), mtime3)
+	checkTime(fmt.Sprintf("%x-d", entry.OutputID), mtime3)
+
+	// Another half a day later, Trim should actually run, and it should remove id.
+	now += 86400/2 + 1
+	c.Trim()
+	if _, err := c.Get(dummyID(1)); err == nil {
+		t.Fatal("Trim did not remove dummyID(1)")
+	}
+}
diff --git a/src/cmd/go/internal/work/exec.go b/src/cmd/go/internal/work/exec.go
index 43409de764..fc4a36ddf4 100644
--- a/src/cmd/go/internal/work/exec.go
+++ b/src/cmd/go/internal/work/exec.go
@@ -53,6 +53,11 @@ func actionList(root *Action) []*Action {
 
 // do runs the action graph rooted at root.
 func (b *Builder) Do(root *Action) {
+	if c := cache.Default(); c != nil && !b.ComputeStaleOnly {
+		// If we're doing real work, take time at the end to trim the cache.
+		defer c.Trim()
+	}
+
 	// Build list of all actions, assigning depth-first post-order priority.
 	// The original implementation here was a true queue
 	// (using a channel) but it had the effect of getting