tka: stable text representation of AUMHash

This makes debugging easier, you can pass an AUMHash to a printf and get
a string that is easy to debug.

Also rearrange how directories/files work in the FS store: use the first
two characters of the string representation as the prefix directory, and
use the entire AUMHash string as the file name. This is again to aid
debugging: you can `ls` a directory and line up what prints out easily
with what you get from a printf in debug code.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

tka/tailchonk.go

@@ -6,8 +6,6 @@ package tka
 
 import (
 	"bytes"
-	"encoding/base32"
-	"encoding/hex"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -190,7 +188,8 @@ func ChonkDir(dir string) (*FS, error) {
 // fsHashInfo describes how information about an AUMHash is represented
 // on disk.
 //
-// The CBOR-serialization of this struct is stored to base/hex(hash[0])/base32(hash[1:])
+// The CBOR-serialization of this struct is stored to base/__/base32(hash)
+// where __ are the first two characters of base32(hash).
 //
 // CBOR was chosen because we are already using it and it serializes
 // much smaller than JSON for AUMs. The 'keyasint' thing isn't essential
@@ -200,12 +199,11 @@ type fsHashInfo struct {
 	AUM      *AUM      `cbor:"2,keyasint"`
 }
 
-func (c *FS) dirPrefix(h AUMHash) string {
-	return filepath.Join(c.base, hex.EncodeToString(h[:1]))
-}
-
-func (c *FS) filename(h AUMHash) string {
-	return base32.StdEncoding.WithPadding(base32.NoPadding).EncodeToString(h[1:])
+// aumDir returns the directory an AUM is stored in, and its filename
+// within the directory.
+func (c *FS) aumDir(h AUMHash) (dir, base string) {
+	s := h.String()
+	return filepath.Join(c.base, s[:2]), s
 }
 
 // AUM returns the AUM with the specified digest.
@@ -256,7 +254,8 @@ func (c *FS) ChildAUMs(prevAUMHash AUMHash) ([]AUM, error) {
 }
 
 func (c *FS) get(h AUMHash) (*fsHashInfo, error) {
-	f, err := os.Open(filepath.Join(c.dirPrefix(h), c.filename(h)))
+	dir, base := c.aumDir(h)
+	f, err := os.Open(filepath.Join(dir, base))
 	if err != nil {
 		return nil, err
 	}
@@ -266,6 +265,9 @@ func (c *FS) get(h AUMHash) (*fsHashInfo, error) {
 	if err := cbor.NewDecoder(f).Decode(&out); err != nil {
 		return nil, err
 	}
+	if out.AUM != nil && out.AUM.Hash() != h {
+		return nil, fmt.Errorf("%s: AUM does not match file name hash %s", f.Name(), out.AUM.Hash())
+	}
 	return &out, nil
 }
 
@@ -297,24 +299,15 @@ func (c *FS) scanHashes(eachHashInfo func(*fsHashInfo)) error {
 		if !prefix.IsDir() {
 			continue
 		}
-		pb, err := hex.DecodeString(prefix.Name())
-		if err != nil || len(pb) != 1 {
-			return fmt.Errorf("invalid prefix directory %q: %v", prefix.Name(), err)
-		}
-
 		files, err := os.ReadDir(filepath.Join(c.base, prefix.Name()))
 		if err != nil {
 			return fmt.Errorf("reading prefix dir: %v", err)
 		}
 		for _, file := range files {
-			remainingHash, err := base32.StdEncoding.WithPadding(base32.NoPadding).DecodeString(file.Name())
-			if err != nil {
-				return fmt.Errorf("invalid aum file %s/%s: %v", prefix.Name(), file.Name(), err)
-			}
 			var h AUMHash
-			h[0] = pb[0]
-			copy(h[1:], remainingHash)
-
+			if err := h.UnmarshalText([]byte(file.Name())); err != nil {
+				return fmt.Errorf("invalid aum file: %s: %w", file.Name(), err)
+			}
 			info, err := c.get(h)
 			if err != nil {
 				return fmt.Errorf("reading %x: %v", h, err)
@@ -422,7 +415,8 @@ func (c *FS) commit(h AUMHash, updater func(*fsHashInfo)) error {
 		return fmt.Errorf("cannot commit AUM with hash %x to %x", toCommit.AUM.Hash(), h)
 	}
 
-	if err := os.MkdirAll(c.dirPrefix(h), 0755); err != nil && !os.IsExist(err) {
+	dir, base := c.aumDir(h)
+	if err := os.MkdirAll(dir, 0755); err != nil && !os.IsExist(err) {
 		return fmt.Errorf("creating directory: %v", err)
 	}
 
@@ -430,5 +424,5 @@ func (c *FS) commit(h AUMHash, updater func(*fsHashInfo)) error {
 	if err := cbor.NewEncoder(&buff).Encode(toCommit); err != nil {
 		return fmt.Errorf("encoding: %v", err)
 	}
-	return atomicfile.WriteFile(filepath.Join(c.dirPrefix(h), c.filename(h)), buff.Bytes(), 0644)
+	return atomicfile.WriteFile(filepath.Join(dir, base), buff.Bytes(), 0644)
 }