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>

src/dist/component/package.rs

@@ -182,9 +182,23 @@ fn unpack_ram(
         .ok()
         .and_then(|budget_str| budget_str.parse::<usize>().ok())
     {
-        // Note: In future we may want to add a warning or even an override if a user
-        // supplied budget is larger than effective_max_ram.
-        Some(budget) => budget,
+        Some(budget) => {
+            if budget < minimum_ram {
+                warn!(
+                    "Specified unpack RAM limit ({}) less than minimum of {}.  Unpacking may fail.",
+                    budget, minimum_ram
+                );
+            }
+            if budget > default_max_unpack_ram {
+                warn!(
+                    "Specified unpack RAM limit ({}) greater than detected available RAM of {}.  Clamping to detected limit.",
+                    budget, default_max_unpack_ram
+                );
+                default_max_unpack_ram
+            } else {
+                budget
+            }
+        }
         None => {
             if let Some(h) = notify_handler {
                 h(Notification::SetDefaultBufferSize(default_max_unpack_ram))
@@ -193,10 +207,11 @@ 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 {
+        minimum_ram
+    } else {
+        unpack_ram
     }
-    unpack_ram
 }
 
 /// Handle the async result of io operations