mach: Improve test-speedometer error reporting (#38887)

Currently speedometer results are parsed by parsing the console output
from stdout (or a file in the case of ohos). Currently json decode
errors just cause mach to crash. Instead print an error message, point
to the problematic location and exit.
A crash can happen if something else also prints, e.g. on macos, we have
the `xx threads are still running` message at shutdown. Hence this PR
doesn't really fix the unreliable nature of the current implementation,
but at least adds a helpful error message, which would point people in
the right direction.

Testing: test-speedometer is run in CI

---------

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

python/servo/testing_commands.py

@@ -113,6 +113,22 @@ def format_with_rustfmt(check_only: bool = True) -> int:
     )
 
 
+def pretty_print_json_decode_error(error: json.decoder.JSONDecodeError) -> None:
+    print(f"json decode error: {error}")
+    # Print section around that character that raised the json decode error.
+    snippet_radius = 25
+    start, stop = max(0, error.pos - snippet_radius), error.pos + snippet_radius
+    snippet = error.doc[start:stop]
+    print("```")
+    prefix = "... " if start != 0 else ""
+    suffix = " ..." if stop < len(error.doc) else ""
+    print(prefix, snippet, suffix, sep="")
+    # If the snippet is multiline, this won't work as expected, but it's a best effort.
+    right_justification = snippet_radius + 1 + len(suffix)
+    print("^ Offending character".rjust(right_justification))
+    print("```")
+
+
 @CommandProvider
 class MachCommands(CommandBase):
     DEFAULT_RENDER_MODE = "cpu"
@@ -617,8 +633,7 @@ class MachCommands(CommandBase):
             json.dump(output, f, indent=4)
 
     def speedometer_runner(self, binary: str, bmf_output: str | None) -> None:
-        speedometer = json.loads(
-            subprocess.check_output(
+        output = subprocess.check_output(
             [
                 binary,
                 "https://servospeedometer.netlify.app?headless=1",
@@ -627,9 +642,16 @@ class MachCommands(CommandBase):
                 "--window-size=1100x900",
                 "--headless",
             ],
+            encoding="utf-8",
             timeout=120,
-            ).decode()
         )
+        try:
+            speedometer = json.loads(output)
+        except json.decoder.JSONDecodeError as e:
+            pretty_print_json_decode_error(e)
+            print("Error: Failed to parse speedometer results")
+            print("This can happen if other log messages are printed while running servo...")
+            exit(1)
 
         print(f"Score: {speedometer['Score']['mean']} ± {speedometer['Score']['delta']}")
 
@@ -646,13 +668,13 @@ class MachCommands(CommandBase):
             hdc_path = path.join(ohos_sdk_native, "../", "toolchains", "hdc")
 
         def read_log_file(hdc_path: str) -> str:
-            subprocess.call([hdc_path, "file", "recv", log_path])
+            subprocess.call([hdc_path, "file", "recv", log_path, "servo.log"])
             file = ""
             try:
-                file = open("servo.log")
+                with open("servo.log") as file:
+                    return file.read()
             except OSError:
                 return ""
-            return file.read()
 
         subprocess.call([hdc_path, "shell", "aa", "force-stop", "org.servo.servo"])
 
@@ -681,9 +703,6 @@ class MachCommands(CommandBase):
 
         # A current (2025-06-23) run took 3m 49s = 229s. We keep a safety margin
         # but we will exit earlier if we see "{"
-        # Currently ohos has a bug where the event loop gets stuck. We produce a
-        # touch event every minute to prevent this
-        # See https://github.com/servo/servo/issues/37727
         whole_file: str = ""
         for i in range(10):
             sleep(30)
@@ -694,9 +713,21 @@ class MachCommands(CommandBase):
                 sleep(2)
                 whole_file = read_log_file(hdc_path)
                 break
+        else:
+            print("Error failed to find console logs in log file")
+            print(f"log-file contents: `{whole_file}`")
+            exit(1)
         start_index: int = whole_file.index("[INFO script::dom::console]") + len("[INFO script::dom::console]") + 1
         json_string = whole_file[start_index:]
+        try:
             speedometer = json.loads(json_string)
+        except json.decoder.JSONDecodeError as e:
+            print(f"Error: Failed to convert log output to JSON: {e}")
+            pretty_print_json_decode_error(e)
+            print("Error: Failed to parse speedometer results")
+            print("This can happen if other log messages are printed while running servo...")
+            exit(1)
+
         print(f"Score: {speedometer['Score']['mean']} ± {speedometer['Score']['delta']}")
         if bmf_output:
             self.speedometer_to_bmf(speedometer, bmf_output, profile)