compress: treat an empty BORG_ZSTD_MT_WORKERS as unset

docs/usage/general/environment.rst.inc

@@ -225,7 +225,8 @@ General:
         0 means "always multi-threaded", a very large value effectively disables multi-threading.
     BORG_ZSTD_MT_WORKERS
         When set to a numeric value, use that many threads to zstd-compress a single chunk
-        (default: the cpu count, but at most 4). 0 or 1 means single-threaded compression.
+        (default: the cpu count, but at most 4). If it is unset or empty, the default is
+        used. 0 or 1 means single-threaded compression.
         Only relevant when compressing with ``zstd``.
         Chunks below 768KiB are always compressed single-threaded: libzstd will not use a
         compression job smaller than 512KiB, so a small chunk gets split very unevenly and

src/borg/archiver/help_cmd.py

@@ -819,7 +819,8 @@ class HelpMixIn:
                 0 means "always multi-threaded", a very large value effectively disables multi-threading.
             BORG_ZSTD_MT_WORKERS
                 When set to a numeric value, use that many threads to zstd-compress a single chunk
-                (default: the cpu count, but at most 4). 0 or 1 means single-threaded compression.
+                (default: the cpu count, but at most 4). If it is unset or empty, the default is
+                used. 0 or 1 means single-threaded compression.
                 Only relevant when compressing with ``zstd``.
                 Chunks below 768KiB are always compressed single-threaded: libzstd will not use a
                 compression job smaller than 512KiB, so a small chunk gets split very unevenly and

src/borg/compress.pyx

@@ -59,7 +59,8 @@ _zstd_mt_workers = None  # cached (chunk workers, stream workers)
 def get_zstd_mt_workers(stream=False):
     """How many threads libzstd may use to compress a single chunk (or stream).
 
-    BORG_ZSTD_MT_WORKERS overrides the defaults below (for chunks and streams alike).
+    BORG_ZSTD_MT_WORKERS overrides the defaults below (for chunks and streams alike);
+    if it is unset or empty, the defaults are used.
     0 or 1 means single-threaded compression, which also avoids the small loss of
     compression ratio that splitting a chunk into jobs causes (measured at zstd,3:
     +0.05% for a 1MiB chunk, +0.64% for an 8MiB one; higher levels lose a bit more as
@@ -84,9 +85,9 @@ def get_zstd_mt_workers(stream=False):
     """
     global _zstd_mt_workers
     if _zstd_mt_workers is None:
-        value = os.environ.get("BORG_ZSTD_MT_WORKERS")
+        value = os.environ.get("BORG_ZSTD_MT_WORKERS", "").strip()
         cpus = os.cpu_count() or 1
-        if value is None:
+        if not value:  # unset or empty: use the defaults
             workers = (min(cpus, 4), cpus)
         else:
             try:

src/borg/testsuite/compress_test.py

@@ -350,9 +350,12 @@ def workers_for(env_value, stream=False):
     cpus = os.cpu_count() or 1
     assert workers_for(None) == min(cpus, 4)  # per-chunk default is capped
     assert workers_for(None, stream=True) == cpus  # stream default is not
+    for empty in ["", " "]:  # an empty value is treated like an unset one
+        assert workers_for(empty) == min(cpus, 4)
+        assert workers_for(empty, stream=True) == cpus
     for value, expected in [("0", 0), ("1", 1), ("4", 4), ("12", 12)]:
         assert workers_for(value) == expected  # the env var is not capped
         assert workers_for(value, stream=True) == expected
-    for invalid in ["", "yes", "4x", "1.5", "-1"]:
+    for invalid in ["yes", "4x", "1.5", "-1"]:
         with pytest.raises(Error):
             workers_for(invalid)