aarch64: use single qemu-img thread

Work around a bug on aarch64[1] where `qemu-img` would hang
about a third of the time when converting images. To be able 
to activate the work-around based on the environment, i.e.
only on certain distributions, introduce an environment
variable, `OSBUILD_QEMU_IMG_COROUTINES`, that is set in the
runner and then picked up in the assembler.

[1] https://bugs.launchpad.net/qemu/+bug/1805256

assemblers/org.osbuild.qemu

@@ -699,6 +699,11 @@ def main(tree, output_dir, options, loop_client):
         if compat:
             extra_args["qcow2"] += ["-o", f"compat={compat}"]
 
+        coroutines = os.environ.get("OSBUILD_QEMU_IMG_COROUTINES")
+        if coroutines:
+            print(f"qemu-img coroutines: {coroutines}")
+            extra_args[fmt] += ["-m", coroutines]
+
         subprocess.run([
             "qemu-img",
             "convert",

runners/org.osbuild.rhel82

@@ -1,12 +1,26 @@
 #!/usr/libexec/platform-python
 
 import os
+import platform
 import subprocess
 import sys
 
 import osbuild.api
 
 
+def quirks():
+    # Platform specific quirks
+    env = os.environ.copy()
+
+    if platform.machine() == "aarch64":
+        # Work around a bug in qemu-img on aarch64 that can lead to qemu-img
+        # hangs when more then one coroutine is use (which is the default)
+        # See https://bugs.launchpad.net/qemu/+bug/1805256
+        env["OSBUILD_QEMU_IMG_COROUTINES"] = "1"
+
+    return env
+
+
 def ldconfig():
     # ld.so.conf must exist, or `ldconfig` throws a warning
     subprocess.run(["touch", "/etc/ld.so.conf"], check=True)
@@ -57,7 +71,11 @@ def main():
         nsswitch()
         python_alternatives()
 
-        r = subprocess.run(sys.argv[1:], check=False)
+        env = quirks()
+
+        r = subprocess.run(sys.argv[1:],
+                           env=env,
+                           check=False)
         sys.exit(r.returncode)