Auto merge of #15026 - tydus101:max-session-fix, r=jdm

Max session fix

<!-- Please describe your changes on the following line: -->
Converted document discarding from an opt into a pref. Updated testing docs and changed all uses to pref api.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14960 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because current test suite is sufficient

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4 files changed, 4 insertions, 12 deletions

diff --git a/components/config/opts.rs b/components/config/opts.rs
index 32195664f2d..df2bb2e041e 100644
--- a/components/config/opts.rs
+++ b/components/config/opts.rs
@@ -68,9 +68,6 @@ pub struct Opts {
 
     pub output_file: Option<String>,
 
-    /// How much session history to keep in each tab.
-    pub max_session_history: usize,
-
     /// Replace unpaires surrogates in DOM strings with U+FFFD.
     /// See https://github.com/servo/servo/issues/6564
     pub replace_surrogates: bool,
@@ -521,7 +518,6 @@ pub fn default_opts() -> Opts {
         userscripts: None,
         user_stylesheets: Vec::new(),
         output_file: None,
-        max_session_history: 20,
         replace_surrogates: false,
         gc_profile: false,
         load_webfonts_synchronously: false,
@@ -615,7 +611,6 @@ pub fn from_cmdline_args(args: &[String]) -> ArgumentParsingResult {
                 "Probability of randomly closing a pipeline (for testing constellation hardening).",
                 "0.0");
     opts.optopt("", "random-pipeline-closure-seed", "A fixed seed for repeatbility of random pipeline closure.", "");
-    opts.optopt("", "max-session-history", "Maximum amount of session history to store in each tab.", "20");
     opts.optmulti("Z", "debug",
                   "A comma-separated string of debug options. Pass help to show available options.", "");
     opts.optflag("h", "help", "Print this message");
@@ -784,10 +779,6 @@ pub fn from_cmdline_args(args: &[String]) -> ArgumentParsingResult {
         }
     };
 
-    let max_session_history = opt_match.opt_str("max-session-history").map(|max| {
-        max.parse().unwrap_or_else(|err| args_fail(&format!("Error parsing option: --max-session-history ({})", err)))
-    }).unwrap_or(20);
-
     if opt_match.opt_present("M") {
         MULTIPROCESS.store(true, Ordering::SeqCst)
     }
@@ -829,7 +820,6 @@ pub fn from_cmdline_args(args: &[String]) -> ArgumentParsingResult {
         userscripts: opt_match.opt_default("userscripts", ""),
         user_stylesheets: user_stylesheets,
         output_file: opt_match.opt_str("o"),
-        max_session_history: max_session_history,
         replace_surrogates: debug_options.replace_surrogates,
         gc_profile: debug_options.gc_profile,
         load_webfonts_synchronously: debug_options.load_webfonts_synchronously,
diff --git a/components/constellation/constellation.rs b/components/constellation/constellation.rs
index fc67095dce7..15c0e258ea3 100644
--- a/components/constellation/constellation.rs
+++ b/components/constellation/constellation.rs
@@ -2136,7 +2136,8 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
         } else if let Some(frame) = self.frames.get_mut(&frame_change.frame_id) {
             debug!("Adding pipeline to existing frame.");
             frame.load(frame_change.new_pipeline_id, frame_change.url.clone());
-            let evicted_id = frame.prev.len().checked_sub(opts::get().max_session_history)
+            let evicted_id = frame.prev.len()
+                .checked_sub(PREFS.get("session-history.max-length").as_u64().unwrap_or(20) as usize)
                 .and_then(|index| frame.prev.get_mut(index))
                 .and_then(|entry| entry.pipeline_id.take());
             (evicted_id, false, true, true)
diff --git a/resources/prefs.json b/resources/prefs.json
index 67fea25b136..74c8fa7f9fe 100644
--- a/resources/prefs.json
+++ b/resources/prefs.json
@@ -56,6 +56,7 @@
   "layout.viewport.enabled": false,
   "layout.writing-mode.enabled": false,
   "network.mime.sniff": false,
+  "session-history.max-length": 20,
   "shell.builtin-key-shortcuts.enabled": true,
   "shell.homepage": "https://servo.org",
   "shell.native-titlebar.enabled": true
diff --git a/tests/wpt/mozilla/tests/mozilla/servo-max-session-history.html b/tests/wpt/mozilla/tests/mozilla/servo-max-session-history.html
index 7d60d8f3f9f..a137051a578 100644
--- a/tests/wpt/mozilla/tests/mozilla/servo-max-session-history.html
+++ b/tests/wpt/mozilla/tests/mozilla/servo-max-session-history.html
@@ -18,7 +18,7 @@
   var go_forward_by = 24;
 
   // The number of pages to go back by.
-  // This should be more than the default --max-session-history,
+  // This should be more than the default session-history.max-length pref,
   // to ensure that going back reloads the page.
   var go_back_by = Math.min(page_number, 20);