sd-journal: allow to specify compression algorithm through env

Fixes RHBZ#2183546 (https://bugzilla.redhat.com/show_bug.cgi?id=2183546).

Previously, journal file is always compressed with the default algorithm
set at compile time. So, if a newer algorithm is used, journal files
cannot be read by older version of journalctl that does not support the
algorithm.

Co-authored-by: Colin Walters <walters@verbum.org>

docs/ENVIRONMENT.md

@@ -513,6 +513,14 @@ SYSTEMD_HOME_DEBUG_SUFFIX=foo \
   journal. Note that journal files in compact mode are limited to 4G to allow use of
   32-bit offsets. Enabled by default.
 
+* `$SYSTEMD_JOURNAL_COMPRESS` – Takes a boolean, or one of the compression
+  algorithms "XZ", "LZ4", and "ZSTD". If enabled, the default compression
+  algorithm set at compile time will be used when opening a new journal file.
+  If disabled, the journal file compression will be disabled. Note that the
+  compression mode of existing journal files are not changed. To make the
+  specified algorithm takes an effect immediately, you need to explicitly run
+  `journalctl --rotate`.
+
 `systemd-pcrphase`, `systemd-cryptsetup`:
 
 * `$SYSTEMD_FORCE_MEASURE=1` — If set, force measuring of resources (which are

src/libsystemd/sd-journal/journal-file.c

@@ -347,6 +347,48 @@ static bool compact_mode_requested(void) {
         return cached;
 }
 
+#if HAVE_COMPRESSION
+static Compression getenv_compression(void) {
+        Compression c;
+        const char *e;
+        int r;
+
+        e = getenv("SYSTEMD_JOURNAL_COMPRESS");
+        if (!e)
+                return DEFAULT_COMPRESSION;
+
+        r = parse_boolean(e);
+        if (r >= 0)
+                return r ? DEFAULT_COMPRESSION : COMPRESSION_NONE;
+
+        c = compression_from_string(e);
+        if (c < 0) {
+                log_debug_errno(c, "Failed to parse SYSTEMD_JOURNAL_COMPRESS value, ignoring: %s", e);
+                return DEFAULT_COMPRESSION;
+        }
+
+        if (!compression_supported(c)) {
+                log_debug("Unsupported compression algorithm specified, ignoring: %s", e);
+                return DEFAULT_COMPRESSION;
+        }
+
+        return c;
+}
+#endif
+
+static Compression compression_requested(void) {
+#if HAVE_COMPRESSION
+        static thread_local Compression cached = _COMPRESSION_INVALID;
+
+        if (cached < 0)
+                cached = getenv_compression();
+
+        return cached;
+#else
+        return COMPRESSION_NONE;
+#endif
+}
+
 static int journal_file_init_header(
                 JournalFile *f,
                 JournalFileFlags file_flags,
@@ -366,7 +408,7 @@ static int journal_file_init_header(
         Header h = {
                 .header_size = htole64(ALIGN64(sizeof(h))),
                 .incompatible_flags = htole32(
-                                FLAGS_SET(file_flags, JOURNAL_COMPRESS) * COMPRESSION_TO_HEADER_INCOMPATIBLE_FLAG(DEFAULT_COMPRESSION) |
+                                FLAGS_SET(file_flags, JOURNAL_COMPRESS) * COMPRESSION_TO_HEADER_INCOMPATIBLE_FLAG(compression_requested()) |
                                 keyed_hash_requested() * HEADER_INCOMPATIBLE_KEYED_HASH |
                                 compact_mode_requested() * HEADER_INCOMPATIBLE_COMPACT),
                 .compatible_flags = htole32(