tka: improve logging for Compact and Commit operations

Log whenever we:

* Commit an AUM which was previously soft-deleted (which we don't expect
  to happen in practice, and may indicate an issue with our sync code)
* Purge AUMs during a Compact operation.
* Successfully commit AUMs as part of a bootstrap or sync operation.

All three logs mention `tka` for easy of discoverability.

Updates tailscale/corp#39455

Change-Id: I2b07bb0ef075877f40ec34b80bb668be59e1cdc3
Signed-off-by: Alex Chan <alexc@tailscale.com>

tka/tka.go

@@ -10,6 +10,7 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"log"
 	"os"
 	"sort"
 
@@ -556,6 +557,8 @@ func Bootstrap(storage Chonk, bootstrap AUM) (*Authority, error) {
 	// Everything looks good, write it to storage.
 	if err := storage.CommitVerifiedAUMs([]AUM{bootstrap}); err != nil {
 		return nil, fmt.Errorf("commit: %v", err)
+	} else {
+		log.Printf("tka.Bootstrap: successfully committed bootstrap AUM (%s)", bootstrap.Hash())
 	}
 	if err := storage.SetLastActiveAncestor(bootstrap.Hash()); err != nil {
 		return nil, fmt.Errorf("set ancestor: %v", err)
@@ -587,6 +590,7 @@ func (a *Authority) InformIdempotent(storage Chonk, updates []AUM) (Authority, e
 	}
 	stateAt := make(map[AUMHash]State, len(updates)+1)
 	toCommit := make([]AUM, 0, len(updates))
+	toCommitHashes := make([]AUMHash, 0, len(updates))
 	prevHash := a.Head()
 
 	// The state at HEAD is the current state of the authority. It's likely
@@ -636,10 +640,13 @@ func (a *Authority) InformIdempotent(storage Chonk, updates []AUM) (Authority, e
 		}
 		prevHash = hash
 		toCommit = append(toCommit, update)
+		toCommitHashes = append(toCommitHashes, update.Hash())
 	}
 
 	if err := storage.CommitVerifiedAUMs(toCommit); err != nil {
 		return Authority{}, fmt.Errorf("commit: %v", err)
+	} else {
+		log.Printf("tka.CommitVerifiedAUMs: successfully committed %d AUMs: %v", len(toCommit), toCommitHashes)
 	}
 
 	if isHeadChain {