Support compression for Actions logs (#31761)

Support compression for Actions logs to save storage space and
bandwidth. Inspired by
https://github.com/go-gitea/gitea/issues/24256#issuecomment-1521153015

The biggest challenge is that the compression format should support
[seekable](https://github.com/facebook/zstd/blob/dev/contrib/seekable_format/zstd_seekable_compression_format.md).
So when users are viewing a part of the log lines, Gitea doesn't need to
download the whole compressed file and decompress it.

That means gzip cannot help here. And I did research, there aren't too
many choices, like bgzip and xz, but I think zstd is the most popular
one. It has an implementation in Golang with
[zstd](https://github.com/klauspost/compress/tree/master/zstd) and
[zstd-seekable-format-go](https://github.com/SaveTheRbtz/zstd-seekable-format-go),
and what is better is that it has good compatibility: a seekable format
zstd file can be read by a regular zstd reader.

This PR introduces a new package `zstd` to combine and wrap the two
packages, to provide a unified and easy-to-use API.

And a new setting `LOG_COMPRESSION` is added to the config, although I
don't see any reason why not to use compression, I think's it's a good
idea to keep the default with `none` to be consistent with old versions.

`LOG_COMPRESSION` takes effect for only new log files, it adds `.zst` as
an extension to the file name, so Gitea can determine if it needs
decompression according to the file name when reading. Old files will
keep the format since it's not worth converting them, as they will be
cleared after #31735.

<img width="541" alt="image"
src="https://github.com/user-attachments/assets/e9598764-a4e0-4b68-8c2b-f769265183c9">

modules/actions/log.go

@@ -15,6 +15,7 @@ import (
 	"code.gitea.io/gitea/models/dbfs"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/storage"
+	"code.gitea.io/gitea/modules/zstd"
 
 	runnerv1 "code.gitea.io/actions-proto-go/runner/v1"
 	"google.golang.org/protobuf/types/known/timestamppb"
@@ -28,6 +29,9 @@ const (
 	defaultBufSize = MaxLineSize
 )
 
+// WriteLogs appends logs to DBFS file for temporary storage.
+// It doesn't respect the file format in the filename like ".zst", since it's difficult to reopen a closed compressed file and append new content.
+// Why doesn't it store logs in object storage directly? Because it's not efficient to append content to object storage.
 func WriteLogs(ctx context.Context, filename string, offset int64, rows []*runnerv1.LogRow) ([]int, error) {
 	flag := os.O_WRONLY
 	if offset == 0 {
@@ -106,6 +110,17 @@ func ReadLogs(ctx context.Context, inStorage bool, filename string, offset, limi
 	return rows, nil
 }
 
+const (
+	// logZstdBlockSize is the block size for zstd compression.
+	// 128KB leads the compression ratio to be close to the regular zstd compression.
+	// And it means each read from the underlying object storage will be at least 128KB*(compression ratio).
+	// The compression ratio is about 30% for text files, so the actual read size is about 38KB, which should be acceptable.
+	logZstdBlockSize = 128 * 1024 // 128KB
+)
+
+// TransferLogs transfers logs from DBFS to object storage.
+// It happens when the file is complete and no more logs will be appended.
+// It respects the file format in the filename like ".zst", and compresses the content if needed.
 func TransferLogs(ctx context.Context, filename string) (func(), error) {
 	name := DBFSPrefix + filename
 	remove := func() {
@@ -119,7 +134,26 @@ func TransferLogs(ctx context.Context, filename string) (func(), error) {
 	}
 	defer f.Close()
 
-	if _, err := storage.Actions.Save(filename, f, -1); err != nil {
+	var reader io.Reader = f
+	if strings.HasSuffix(filename, ".zst") {
+		r, w := io.Pipe()
+		reader = r
+		zstdWriter, err := zstd.NewSeekableWriter(w, logZstdBlockSize)
+		if err != nil {
+			return nil, fmt.Errorf("zstd NewSeekableWriter: %w", err)
+		}
+		go func() {
+			defer func() {
+				_ = w.CloseWithError(zstdWriter.Close())
+			}()
+			if _, err := io.Copy(zstdWriter, f); err != nil {
+				_ = w.CloseWithError(err)
+				return
+			}
+		}()
+	}
+
+	if _, err := storage.Actions.Save(filename, reader, -1); err != nil {
 		return nil, fmt.Errorf("storage save %q: %w", filename, err)
 	}
 	return remove, nil
@@ -150,11 +184,22 @@ func OpenLogs(ctx context.Context, inStorage bool, filename string) (io.ReadSeek
 		}
 		return f, nil
 	}
+
 	f, err := storage.Actions.Open(filename)
 	if err != nil {
 		return nil, fmt.Errorf("storage open %q: %w", filename, err)
 	}
-	return f, nil
+
+	var reader io.ReadSeekCloser = f
+	if strings.HasSuffix(filename, ".zst") {
+		r, err := zstd.NewSeekableReader(f)
+		if err != nil {
+			return nil, fmt.Errorf("zstd NewSeekableReader: %w", err)
+		}
+		reader = r
+	}
+
+	return reader, nil
 }
 
 func FormatLog(timestamp time.Time, content string) string {