[Webkit-unassigned] [Bug 148680] New: Web Inspector: add explicit version checking for legacy backends

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=148680

            Bug ID: 148680
           Summary: Web Inspector: add explicit version checking for
                    legacy backends
    Classification: Unclassified
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Web Inspector
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: bburg at apple.com
                CC: bburg at apple.com, graouts at webkit.org,
                    joepeck at webkit.org, mattbaker at apple.com,
                    nvasilyev at apple.com, timothy at apple.com,
                    webkit-bug-importer at group.apple.com

In several places we do awkward things like the following since we can't directly test protocol version or supported features.

// COMPATIBILITY (iOS 9): Legacy backends don't support breakpoint ignore count. Since support
// can't be tested directly, check for CSS.getSupportedSystemFontFamilyNames.
if (CSSAgent.getSupportedSystemFontFamilyNames) {

In cases where we can't use Agent.command.supports(...), it would be better if we had checks against the shipped protocol version, like:

if (WebInspector.backendVersion.builtBefore(WebInspector.BackendVersions.iOS9))
if (WebInspector.backendVersion.equalTo(WebInspector.BackendVersions.iOS8))
if (WebInspector.backendVersion.builtAfter(WebInspector.BackendVersions.iOS6))

Feel free to suggest better comparison operator names, these are clunky. I believe WebKit uses "builtOnOrAfter(10, 10, 0)" or similar.
In the legacy protocol versions, we can embed a top-level "version" field and parse it in the frontend. And, going forward, we can use
a version string of iOS9+ once a legacy copy of iOS9 has been copied over.

We could also implement fine-grained feature checking, like WebInspector.backendVersion.supports("Debugger.BreakpointIgnoreCounts"), which will return false for old versions; once that specific feature support lands in trunk, we add the feature key "Debugger.BreakpointIgnoreCounts" in a file somewhere that feeds into backendVersion.supports(). Once the next legacy protocol version is created, we delete all the old feature key checks and replace them with backendVersion.equalOrLaterThan(new-version).

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