improve: add defensive programming and better error messages for EC

Code review improvements from CodeRabbit:

1. ShardBits Guardrails (ec_volume_info.go):
   - AddShardId, RemoveShardId: Reject shard IDs >= MaxShardCount
   - HasShardId: Return false for out-of-range shard IDs
   - Prevents silent no-ops from bit shifts with invalid IDs

2. Future-Proof Regex (disk_location_ec.go):
   - Updated regex from \.ec[0-9][0-9] to \.ec\d{2,3}
   - Now matches .ec00 through .ec999 (currently .ec00-.ec31 used)
   - Supports future increases to MaxShardCount beyond 99

3. Better Error Messages (volume_grpc_erasure_coding.go):
   - Include valid range (1..32) in dataShards validation error
   - Helps operators quickly identify the problem

4. Validation Before Save (volume_grpc_erasure_coding.go):
   - Validate ECContext (DataShards > 0, ParityShards > 0, Total <= MaxShardCount)
   - Log EC config being saved to .vif for debugging
   - Prevents writing invalid configs to disk

These changes improve robustness and debuggability without changing
core functionality.

weed/server/volume_grpc_erasure_coding.go

@@ -103,11 +103,19 @@ func (vs *VolumeServer) VolumeEcShardsGenerate(ctx context.Context, req *volume_
 	datSize, _, _ := v.FileStat()
 	volumeInfo.DatFileSize = int64(datSize)
 
+	// Validate EC configuration before saving to .vif
+	if ecCtx.DataShards <= 0 || ecCtx.ParityShards <= 0 || ecCtx.Total() > erasure_coding.MaxShardCount {
+		return nil, fmt.Errorf("invalid EC config before saving: data=%d, parity=%d, total=%d (max=%d)",
+			ecCtx.DataShards, ecCtx.ParityShards, ecCtx.Total(), erasure_coding.MaxShardCount)
+	}
+
 	// Save EC configuration to VolumeInfo
 	volumeInfo.EcShardConfig = &volume_server_pb.EcShardConfig{
 		DataShards:   uint32(ecCtx.DataShards),
 		ParityShards: uint32(ecCtx.ParityShards),
 	}
+	glog.V(1).Infof("Saving EC config to .vif for volume %d: %d+%d (total: %d)",
+		req.VolumeId, ecCtx.DataShards, ecCtx.ParityShards, ecCtx.Total())
 
 	if err := volume_info.SaveVolumeInfo(baseFileName+".vif", volumeInfo); err != nil {
 		return nil, fmt.Errorf("SaveVolumeInfo %s: %v", baseFileName, err)
@@ -484,7 +492,7 @@ func (vs *VolumeServer) VolumeEcShardsToVolume(ctx context.Context, req *volume_
 	
 	// Defensive validation to prevent panics from corrupted ECContext
 	if dataShards <= 0 || dataShards > erasure_coding.MaxShardCount {
-		return nil, fmt.Errorf("invalid data shard count %d for volume %d", dataShards, req.VolumeId)
+		return nil, fmt.Errorf("invalid data shard count %d for volume %d (must be 1..%d)", dataShards, req.VolumeId, erasure_coding.MaxShardCount)
 	}
 	
 	shardFileNames := tempShards[:dataShards]

weed/storage/disk_location_ec.go

@@ -16,7 +16,9 @@ import (
 )
 
 var (
-	re = regexp.MustCompile(`\.ec[0-9][0-9]`)
+	// Match .ec00 through .ec999 (currently only .ec00-.ec31 are used)
+	// Using \d{2,3} for future-proofing if MaxShardCount is ever increased beyond 99
+	re = regexp.MustCompile(`\.ec\d{2,3}`)
 )
 
 func (l *DiskLocation) FindEcVolume(vid needle.VolumeId) (*erasure_coding.EcVolume, bool) {

weed/storage/erasure_coding/ec_volume_info.go

@@ -119,14 +119,23 @@ func (ecInfo *EcVolumeInfo) ToVolumeEcShardInformationMessage() (ret *master_pb.
 type ShardBits uint32 // use bits to indicate the shard id, use 32 bits just for possible future extension
 
 func (b ShardBits) AddShardId(id ShardId) ShardBits {
+	if id >= MaxShardCount {
+		return b // Reject out-of-range shard IDs
+	}
 	return b | (1 << id)
 }
 
 func (b ShardBits) RemoveShardId(id ShardId) ShardBits {
+	if id >= MaxShardCount {
+		return b // Reject out-of-range shard IDs
+	}
 	return b &^ (1 << id)
 }
 
 func (b ShardBits) HasShardId(id ShardId) bool {
+	if id >= MaxShardCount {
+		return false // Out-of-range shard IDs are never present
+	}
 	return b&(1<<id) > 0
 }