From 6b784be616ceb999110553d2529721b36c32f93b Mon Sep 17 00:00:00 2001
From: ISSOtm <eldredhabert0@gmail.com>
Date: Sun, 26 Sep 2021 21:54:14 +0200
Subject: [PATCH] Stabilize ties in preproc order through name sort

---
 .../src/format/configuration/preprocessors.md | 10 ++---
 src/book/mod.rs                               | 45 ++++++++++++-------
 2 files changed, 35 insertions(+), 20 deletions(-)

diff --git a/guide/src/format/configuration/preprocessors.md b/guide/src/format/configuration/preprocessors.md
index 37239f98..8d3e1dc6 100644
--- a/guide/src/format/configuration/preprocessors.md
+++ b/guide/src/format/configuration/preprocessors.md
@@ -59,8 +59,8 @@ command = "python random.py"
 
 ### Require A Certain Order
 
-The order in which preprocessors are run is not guaranteed, but you can request some to run before or after others.
-For example, suppose you want your `linenos` preprocessor to process lines that may have been `{{#include}}`d; then you want it to run after the built-in `links` preprocessor, which you can require using the `before` and/or `after` fields.
+The order in which preprocessors are run can be controlled with the `before` and `after` fields.
+For example, suppose you want your `linenos` preprocessor to process lines that may have been `{{#include}}`d; then you want it to run after the built-in `links` preprocessor, which you can require using either the `before` or `after` field:
 
 ```toml
 [preprocessor.linenos]
@@ -74,7 +74,7 @@ or
 before = [ "linenos" ]
 ```
 
-It would be possible, though redundant, to specify both of the above in the same config file.
+It would also be possible, though redundant, to specify both of the above in the same config file.
 
-`mdbook` will detect any infinite loops and error out.
-Note that order of preprocessors besides what is specified using `before` and `after` is not guaranteed, and may change within the same run of `mdbook`.
\ No newline at end of file
+Preprocessors having the same priority specified through `before` and `after` are sorted by name.
+Any infinite loops will be detected and produce an error.
diff --git a/src/book/mod.rs b/src/book/mod.rs
index 33cca5d0..6baac5ca 100644
--- a/src/book/mod.rs
+++ b/src/book/mod.rs
@@ -435,21 +435,36 @@ fn determine_preprocessors(config: &Config) -> Result<Vec<Box<dyn Preprocessor>>
 
     // Now that all links have been established, queue preprocessors in a suitable order
     let mut preprocessors = Vec::with_capacity(preprocessor_names.len());
-    while let Some(name) = preprocessor_names.pop() {
-        let preprocessor: Box<dyn Preprocessor> = match name.as_str() {
-            "links" => Box::new(LinkPreprocessor::new()),
-            "index" => Box::new(IndexPreprocessor::new()),
-            _ => {
-                // The only way to request a custom preprocessor is through the `preprocessor`
-                // table, so it must exist, be a table, and contain the key.
-                let table = &config.get("preprocessor").unwrap().as_table().unwrap()[&name];
-                let command = get_custom_preprocessor_cmd(&name, table);
-                Box::new(CmdPreprocessor::new(name, command))
-            }
-        };
-        preprocessors.push(preprocessor);
+    // `pop_all()` returns an empty vector when no more items are not being depended upon
+    for mut names in std::iter::repeat_with(|| preprocessor_names.pop_all())
+        .take_while(|names| !names.is_empty())
+    {
+        // The `topological_sort` crate does not guarantee a stable order for ties, even across
+        // runs of the same program. Thus, we break ties manually by sorting.
+        // Careful: `str`'s default sorting, which we are implicitly invoking here, uses code point
+        // values ([1]), which may not be an alphabetical sort.
+        // As mentioned in [1], doing so depends on locale, which is not desirable for deciding
+        // preprocessor execution order.
+        // [1]: https://doc.rust-lang.org/stable/std/cmp/trait.Ord.html#impl-Ord-14
+        names.sort();
+        for name in names {
+            let preprocessor: Box<dyn Preprocessor> = match name.as_str() {
+                "links" => Box::new(LinkPreprocessor::new()),
+                "index" => Box::new(IndexPreprocessor::new()),
+                _ => {
+                    // The only way to request a custom preprocessor is through the `preprocessor`
+                    // table, so it must exist, be a table, and contain the key.
+                    let table = &config.get("preprocessor").unwrap().as_table().unwrap()[&name];
+                    let command = get_custom_preprocessor_cmd(&name, table);
+                    Box::new(CmdPreprocessor::new(name, command))
+                }
+            };
+            preprocessors.push(preprocessor);
+        }
     }
 
+    // "If `pop_all` returns an empty vector and `len` is not 0, there are cyclic dependencies."
+    // Normally, `len() == 0` is equivalent to `is_empty()`, so we'll use that.
     if preprocessor_names.is_empty() {
         Ok(preprocessors)
     } else {
@@ -562,8 +577,8 @@ mod tests {
 
         assert!(got.is_ok());
         assert_eq!(got.as_ref().unwrap().len(), 2);
-        assert_eq!(got.as_ref().unwrap()[0].name(), "links");
-        assert_eq!(got.as_ref().unwrap()[1].name(), "index");
+        assert_eq!(got.as_ref().unwrap()[0].name(), "index");
+        assert_eq!(got.as_ref().unwrap()[1].name(), "links");
     }
 
     #[test]