Emit coverage.json timestamp with millisecond precision

The concurrent-overwrite warning compares the timestamp in an existing
`coverage.json` against `PROCESS_START_TIME`. `Time.now` has sub-second
precision, but `iso8601` (no arg) truncates to whole seconds on the way
out, so a sibling writer landing inside the same wall-clock second as
when we started could slip past the check.

Switch `meta.timestamp` to `iso8601(3)` so the round-trip keeps
millisecond fidelity, and update the JSON fixtures and the
`json_formatter_spec` assertion to match.

Author: sferik

CHANGELOG.md

@@ -9,6 +9,7 @@ Unreleased
 * Removed `docile` gem dependency. The `SimpleCov.configure` DSL block is now evaluated via `instance_exec` with instance variable proxying.
 
 ## Enhancements
+* JSON formatter: `meta.timestamp` is now emitted with millisecond precision (`iso8601(3)`) so the concurrent-overwrite warning can distinguish writes within the same wall-clock second
 * JSON formatter: added `total` section with aggregate coverage statistics (covered, missed, total, percent, strength) for line, branch, and method coverage. Line stats additionally include `omitted` (count of blank/comment lines, i.e. lines that cannot be covered)
 * JSON formatter: per-file output now includes `lines_covered_percent`, and when enabled: `branches_covered_percent`, `methods` array, and `methods_covered_percent`
 * JSON formatter: group stats now include full statistics for all enabled coverage types, not just line coverage percent

lib/simplecov/formatter/json_formatter/result_hash_formatter.rb

@@ -93,7 +93,7 @@ def format_meta
             simplecov_version: SimpleCov::VERSION,
             command_name: @result.command_name,
             project_name: SimpleCov.project_name,
-            timestamp: @result.created_at.iso8601,
+            timestamp: @result.created_at.iso8601(3),
             root: SimpleCov.root,
             branch_coverage: SimpleCov.branch_coverage?,
             method_coverage: SimpleCov.method_coverage?

spec/fixtures/json/sample.json

@@ -3,7 +3,7 @@
     "simplecov_version": "0.22.0",
     "command_name": "STUB_COMMAND_NAME",
     "project_name": "STUB_PROJECT_NAME",
-    "timestamp": "2024-01-01T00:00:00+00:00",
+    "timestamp": "2024-01-01T00:00:00.000+00:00",
     "root": "/STUB_WORKING_DIRECTORY",
     "branch_coverage": false,
     "method_coverage": false

spec/fixtures/json/sample_groups.json

@@ -3,7 +3,7 @@
     "simplecov_version": "0.22.0",
     "command_name": "STUB_COMMAND_NAME",
     "project_name": "STUB_PROJECT_NAME",
-    "timestamp": "2024-01-01T00:00:00+00:00",
+    "timestamp": "2024-01-01T00:00:00.000+00:00",
     "root": "/STUB_WORKING_DIRECTORY",
     "branch_coverage": false,
     "method_coverage": false

spec/fixtures/json/sample_with_branch.json

@@ -3,7 +3,7 @@
     "simplecov_version": "0.22.0",
     "command_name": "STUB_COMMAND_NAME",
     "project_name": "STUB_PROJECT_NAME",
-    "timestamp": "2024-01-01T00:00:00+00:00",
+    "timestamp": "2024-01-01T00:00:00.000+00:00",
     "root": "/STUB_WORKING_DIRECTORY",
     "branch_coverage": true,
     "method_coverage": false

spec/fixtures/json/sample_with_method.json

@@ -3,7 +3,7 @@
     "simplecov_version": "0.22.0",
     "command_name": "STUB_COMMAND_NAME",
     "project_name": "STUB_PROJECT_NAME",
-    "timestamp": "2024-01-01T00:00:00+00:00",
+    "timestamp": "2024-01-01T00:00:00.000+00:00",
     "root": "/STUB_WORKING_DIRECTORY",
     "branch_coverage": false,
     "method_coverage": true

spec/json_formatter_spec.rb

@@ -331,7 +331,7 @@
       it "still writes the new file" do
         capture_stderr { subject.format(result) }
 
-        expect(json_output.fetch("meta").fetch("timestamp")).to eq(fixed_time.iso8601)
+        expect(json_output.fetch("meta").fetch("timestamp")).to eq(fixed_time.iso8601(3))
       end
     end