Auto merge of #10616 - autrilla:10614, r=edunham

<!-- Reviewable:start -->
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/10616)
<!-- Reviewable:end -->

python/servo/command_base.py

@@ -103,6 +103,14 @@ def check_call(*args, **kwargs):
     return subprocess.check_call(*args, shell=sys.platform == 'win32', **kwargs)
 
 
+class BuildNotFound(Exception):
+    def __init__(self, message):
+        self.message = message
+
+    def __str__(self):
+        return self.message
+
+
 class CommandBase(object):
     """Base class for mach command providers.
 
@@ -203,6 +211,8 @@ def get_target_dir(self):
             return path.join(self.context.topdir, "target")
 
     def get_binary_path(self, release, dev, android=False):
+        # TODO(autrilla): this function could still use work - it shouldn't
+        # handle quitting, or printing. It should return the path, or an error.
         base_path = self.get_target_dir()
 
         if android:
@@ -220,8 +230,8 @@ def get_binary_path(self, release, dev, android=False):
         dev_exists = path.exists(dev_path)
 
         if not release_exists and not dev_exists:
-            print("No Servo binary found. Please run './mach build' and try again.")
-            sys.exit()
+            raise BuildNotFound('No Servo binary found.'
+                                ' Perhaps you forgot to run `./mach build`?')
 
         if release and release_exists:
             return release_path

python/servo/post_build_commands.py

@@ -23,7 +23,7 @@
     Command,
 )
 
-from servo.command_base import CommandBase, cd, call, check_call
+from servo.command_base import CommandBase, cd, call, check_call, BuildNotFound
 
 
 def read_file(filename, if_exists=False):
@@ -263,11 +263,20 @@ def package(self, release=False, dev=False, debug=False, debugger=None):
     @CommandArgument('--dev', '-d', action='store_true',
                      help='Package the dev build')
     def install(self, release=False, dev=False):
-        binary_path = self.get_binary_path(release, dev, android=True)
-        if not path.exists(binary_path):
-            # TODO: Run the build command automatically?
-            print("Servo build not found. Please run `./mach build` to compile Servo.")
-            return 1
+        try:
+            binary_path = self.get_binary_path(release, dev, android=True)
+        except BuildNotFound:
+            print("Servo build not found. Building servo...")
+            result = Registrar.dispatch(
+                "build", context=self.context, release=release, dev=dev
+            )
+            if result:
+                return result
+            try:
+                binary_path = self.get_binary_path(release, dev, android=True)
+            except BuildNotFound:
+                print("Rebuilding Servo did not solve the missing build problem.")
+                return 1
 
         apk_path = binary_path + ".apk"
         if not path.exists(apk_path):