unpack_ram: More graceful handling of low unpack ram

Some people try setting RUSTUP_UNPACK_RAM below the minimum
RAM needed for a threaded operation.  Since, in principle,
we need at least IO_CHUNK_SIZE and thus in theory 2x IO_CHUNK_SIZE,
this change softens the failure mode from a panic to a warning
and then tries anyway.  This is *likely* to succeed and as such
we'll let it happen.  The warning should make it clear if there's
a problem, and should not appear for no-sysinfo platforms since
they default to minimum_ram anyway.

Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>

Author: kinnison

src/dist/component/package.rs

@@ -193,10 +193,15 @@ fn unpack_ram(
         }
     };
 
-    if io_chunk_size > unpack_ram {
-        panic!("RUSTUP_UNPACK_RAM must be larger than {}", io_chunk_size);
+    if minimum_ram > unpack_ram {
+        warn!(
+            "Available/permitted unpack RAM ({}) less than {}.  Unpacking may fail.",
+            unpack_ram, minimum_ram
+        );
+        minimum_ram
+    } else {
+        unpack_ram
     }
-    unpack_ram
 }
 
 /// Handle the async result of io operations