From 08f73598a5bb3698fa0e0fa0bc4b0037554a9a98 Mon Sep 17 00:00:00 2001
From: dimgigov <gigov@gigov.org>
Date: Thu, 7 May 2026 00:29:43 +0300
Subject: [PATCH] =?UTF-8?q?Fix=20WAL=20crash=20recovery=20=E2=80=94=20data?=
 =?UTF-8?q?=20survives=20power=20loss?=
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- wal.nim: open existing WAL with fmAppend instead of fmWrite (was truncating!)
- lsm.nim: replay WAL entries into memTable on startup before accepting writes
- Fixes real data loss on crash — previously WAL was wiped on every restart
---
 src/barabadb/storage/lsm.nim | 40 +++++++++++++++++++++++++++++++++++-
 src/barabadb/storage/wal.nim | 10 +++++----
 2 files changed, 45 insertions(+), 5 deletions(-)

diff --git a/src/barabadb/storage/lsm.nim b/src/barabadb/storage/lsm.nim
index 268ee48..0e09186 100644
--- a/src/barabadb/storage/lsm.nim
+++ b/src/barabadb/storage/lsm.nim
@@ -10,6 +10,7 @@ import std/locks
 import bloom
 import wal
 import mmap
+import streams
 
 const
   SSTableMagic* = 0x53535442'u32  # "SSTB"
@@ -323,7 +324,44 @@ proc newLSMTree*(dir: string, memMaxSize: int = DefaultMemTableSize): LSMTree =
   result.memMaxSize = memMaxSize
   result.currentSeq = 0
   result.nextSSTableId = nextId
-  discard
+
+  # WAL crash recovery — replay unflushed entries into memTable
+  let walPath = dir / "wal" / "wal.log"
+  if fileExists(walPath):
+    try:
+      let stream = newFileStream(walPath, fmRead)
+      if stream != nil:
+        var magic: uint32 = 0
+        var version: uint32 = 0
+        if stream.readData(addr magic, 4) == 4 and magic == WALMagic:
+          if stream.readData(addr version, 4) == 4:
+            while not stream.atEnd():
+              var kind: uint8 = 0
+              var timestamp: uint64 = 0
+              var keyLen: uint32 = 0
+              var valLen: uint32 = 0
+              if stream.readData(addr kind, 1) != 1: break
+              if stream.readData(addr timestamp, 8) != 8: break
+              if stream.readData(addr keyLen, 4) != 4: break
+              var key = newString(keyLen.int)
+              if keyLen > 0:
+                if stream.readData(addr key[0], keyLen.int) != keyLen.int: break
+              if stream.readData(addr valLen, 4) != 4: break
+              var value = newSeq[byte](valLen.int)
+              if valLen > 0:
+                if stream.readData(addr value[0], valLen.int) != valLen.int: break
+              case WalEntryKind(kind)
+              of wekPut:
+                discard result.memTable.put(key, value, timestamp)
+              of wekDelete:
+                discard result.memTable.put(key, @[], timestamp)  # tombstone
+              of wekCommit:
+                discard
+              of wekCheckpoint:
+                discard
+        stream.close()
+    except:
+      discard
 
 proc put*(db: LSMTree, key: string, value: seq[byte]) =
   acquire(db.lock)
diff --git a/src/barabadb/storage/wal.nim b/src/barabadb/storage/wal.nim
index a70fef2..fa8d48e 100644
--- a/src/barabadb/storage/wal.nim
+++ b/src/barabadb/storage/wal.nim
@@ -28,12 +28,14 @@ type
 proc newWriteAheadLog*(dir: string, syncOnWrite: bool = true): WriteAheadLog =
   createDir(dir)
   let path = dir / "wal.log"
-  let stream = newFileStream(path, fmWrite)
+  let exists = fileExists(path)
+  let stream = if exists: newFileStream(path, fmAppend) else: newFileStream(path, fmWrite)
   if stream == nil:
     raise newException(IOError, "Cannot open WAL: " & path)
-  stream.write(WALMagic)
-  stream.write(WALVersion)
-  stream.flush()
+  if not exists:
+    stream.write(WALMagic)
+    stream.write(WALVersion)
+    stream.flush()
   WriteAheadLog(path: path, stream: stream, entryCount: 0, syncOnWrite: syncOnWrite)
 
 proc writeEntry*(wal: var WriteAheadLog, entry: WalEntry) =