Collapse clean and non clean build requests on clean builders (#266)

Fixes #250 

Where build requests were piling up on slower builders because they were
clean/don't clean/clean/ don't clean etc. and we could not merge those
requests because of that difference.

Galina suggested that if we knew that the builder was going to clean the
build first anyway, we could merge them. This commit implements that.

We do this by first setting a "clean" attribute on the builder's factory
object, that we can find later via in the collapse callback.

So far only ClangBuilder passes this on but any builder can opt in by
copying what it does.

In the collapse function when comparing properties we delete the
"clean_obj" key from both sets of properties if we know that the
builder's factory is always clean.

With some logging (not included in this commit) we can see it now
collapses a clean/non-clean pair of requests:
```
collapseRequests
selfBuildSetProperties: {'scheduler': ('main:clang,clang-tools-extra,compiler-rt,flang,lld,llvm,mlir', 'Scheduler')}
otherBuildSetProperties: {'scheduler': ('main:clang,clang-tools-extra,compiler-rt,flang,lld,llvm,mlir', 'Scheduler'), 'clean_obj': (True, 'Change')}
Removing clean_obj from build set properties.
Build set properties were the same.
```

This has beeen tested by my colleague Omair Javaid on a test build
master. Of course we only tested it with one of Linaro's builders, so
it's possible we see other side effects in production.

Author: DavidSpickett

zorg/buildbot/builders/ClangBuilder.py

@@ -278,7 +278,8 @@ def _getClangCMakeBuildFactory(
     f = LLVMBuildFactory(
             depends_on_projects=depends_on_projects,
             llvm_srcdir='llvm',
-            enable_runtimes=enable_runtimes)
+            enable_runtimes=enable_runtimes,
+            clean=clean)
 
     # Checkout the latest code for LNT
     # and the test-suite separately. Le's do this first,

zorg/buildbot/process/buildrequest.py

@@ -31,6 +31,14 @@ def collapseRequests(master, builder, req1, req2):
             str(req2['buildsetid'])
         )
 
+    # If the build is going to be a clean build anyway, we can collapse a clean
+    # build and a non-clean build.
+    if getattr(builder.config.factory, "clean", False):
+        if 'clean_obj' in selfBuildsetPoperties:
+            del selfBuildsetPoperties["clean_obj"]
+        if 'clean_obj' in otherBuildsetPoperties:
+            del otherBuildsetPoperties["clean_obj"]
+
     # Check buildsets properties and do not collapse
     # if properties do not match. This includes the check
     # for different schedulers.

zorg/buildbot/process/factory.py

@@ -40,6 +40,8 @@ def __init__(self, steps=None, depends_on_projects=None, hint=None, **kwargs):
         BuildFactory.__init__(self, steps)
 
         self.hint = hint
+
+        self.clean = kwargs.pop('clean', False)
 
         # Handle the dependencies.
         if depends_on_projects is None: