allow mamba to set max extract threads

use MAMBA_EXTRACT_THREADS env var in mamba
set max extract threads before transaction
make sure the context value is used by setting context value in fetch_extract_packages
add error handling in mamba in case env var can't be converted to integer

libmamba/src/core/transaction.cpp

@@ -875,6 +875,7 @@ namespace mamba
         Console::instance().init_multi_progress(ProgressBarMode::aggregated);
 
         auto& ctx = Context::instance();
+        DownloadExtractSemaphore::set_max(ctx.extract_threads);
 
         if (ctx.experimental && ctx.verify_artifacts)
             LOG_INFO << "Content trust is enabled, package(s) signatures will be verified";

libmambapy/src/main.cpp

@@ -227,6 +227,7 @@ PYBIND11_MODULE(bindings, m)
         .def_readwrite("local_repodata_ttl", &Context::local_repodata_ttl)
         .def_readwrite("use_index_cache", &Context::use_index_cache)
         .def_readwrite("max_parallel_downloads", &Context::max_parallel_downloads)
+        .def_readwrite("extract_threads", &Context::extract_threads)
         .def_readwrite("always_yes", &Context::always_yes)
         .def_readwrite("dry_run", &Context::dry_run)
         .def_readwrite("ssl_verify", &Context::ssl_verify)

mamba/mamba/utils.py

@@ -211,6 +211,16 @@ def init_api_context(use_mamba_experimental=False):
     api_ctx.channels = context.channels
     api_ctx.platform = context.subdir
 
+    if "MAMBA_EXTRACT_THREADS" in os.environ:
+        try:
+            max_threads = int(os.environ["MAMBA_EXTRACT_THREADS"])
+            api_ctx.extract_threads = max_threads
+        except ValueError:
+            v = os.environ["MAMBA_EXTRACT_THREADS"]
+            raise ValueError(
+                f"Invalid conversion of env variable 'MAMBA_EXTRACT_THREADS' from value '{v}'"
+            )
+
     def get_base_url(url, name=None):
         tmp = url.rsplit("/", 1)[0]
         if name: