Automatically determine edit_link default

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Version `v0.28.0` (unreleased)
 
 * ![Enhancement][badge-enhancement] The `ansicolor` keyword to `HTML()` now defaults to true, meaning that executed outputs from `@example`- and `@repl`-blocks are now by default colored (if they emit colored output). ([#1828][github-1828])
+* ![Enhancement][badge-enhancement] The `edit_link` keyword to `HTML()` automatically tries to figure out if the remote default branch is `main`, `master`, or something else. Documenter will now print a warning if it is unable to unreliably automatically determine `edit_link` or `devbranch` (for `deploydocs`). ([#1827][github-1827], [#1829][github-1829])
 
 ## Version `v0.27.18`
 
@@ -1041,7 +1042,9 @@
 [github-1818]: https://github.com/JuliaDocs/Documenter.jl/pull/1818
 [github-1821]: https://github.com/JuliaDocs/Documenter.jl/pull/1821
 [github-1825]: https://github.com/JuliaDocs/Documenter.jl/pull/1825
+[github-1827]: https://github.com/JuliaDocs/Documenter.jl/issues/1827
 [github-1828]: https://github.com/JuliaDocs/Documenter.jl/pull/1828
+[github-1829]: https://github.com/JuliaDocs/Documenter.jl/pull/1829
 <!-- end of issue link definitions -->
 
 [julia-38079]: https://github.com/JuliaLang/julia/issues/38079

src/Documenter.jl

@@ -532,18 +532,7 @@ function deploydocs(;
 
     # Try to figure out default branch (see #1443 and #1727)
     if devbranch === nothing
-        env = copy(ENV)
-        env["GIT_TERMINAL_PROMPT"] = "0"
-        env["GIT_SSH_COMMAND"] = get(env, "GIT_SSH_COMMAND", "ssh -o \"BatchMode yes\"")
-        str = try
-            read(pipeline(ignorestatus(
-                setenv(`git remote show origin`, env; dir=root)
-            ); stderr=devnull), String)
-        catch
-            ""
-        end
-        m = match(r"^\s*HEAD branch:\s*(.*)$"m, str)
-        devbranch = m === nothing ? "master" : String(m[1])
+        devbranch = Utilities.git_remote_head_branch("deploydocs(devbranch = ...)", root)
     end
 
     deploy_decision = deploy_folder(deploy_config;

src/Utilities/Utilities.jl

@@ -785,6 +785,45 @@ function check_strict_kw(strict)
     return nothing
 end
 
+"""
+Calls `git remote show \$(remotename)` to try to determine the main (development) branch
+of the remote repository. Returns `master` and prints a warning if it was unable to figure
+it out automatically.
+"""
+function git_remote_head_branch(varname, root; remotename = "origin", fallback = "master")
+    env = copy(ENV)
+    env["GIT_TERMINAL_PROMPT"] = "0"
+    env["GIT_SSH_COMMAND"] = get(env, "GIT_SSH_COMMAND", "ssh -o \"BatchMode yes\"")
+    cmd = `git remote show $(remotename)`
+    stderr_output = IOBuffer()
+    git_remote_output = try
+        read(pipeline(setenv(cmd, env; dir=root); stderr=stderr_output), String)
+    catch e
+        @warn """
+        Unable to determine $(varname) from remote HEAD branch, defaulting to "$(fallback)".
+        Calling `git remote` failed with an exception. Set JULIA_DEBUG=Documenter to see the error.
+        Unless this is due to a configuration error, the relevant variable should be set explicitly.
+        """
+        @debug "Command: $cmd" exception = (e, catch_backtrace()) stderr = String(take!(stderr_output))
+        return fallback
+    end
+    m = match(r"^\s*HEAD branch:\s*(.*)$"m, git_remote_output)
+    if m === nothing
+        @warn """
+        Unable to determine $(varname) from remote HEAD branch, defaulting to "$(fallback)".
+        Failed to parse the `git remote` output. Set JULIA_DEBUG=Documenter to see the output.
+        Unless this is due to a configuration error, the relevant variable should be set explicitly.
+        """
+        @debug """
+        stdout from $cmd:
+        $(git_remote_output)
+        """
+        fallback
+    else
+        String(m[1])
+    end
+end
+
 include("DOM.jl")
 include("MDFlatten.jl")
 include("TextDiff.jl")

src/Writers/HTMLWriter.jl

@@ -420,7 +420,7 @@ struct HTML <: Documenter.Writer
     function HTML(;
             prettyurls    :: Bool = true,
             disable_git   :: Bool = false,
-            edit_link     :: Union{String, Symbol, Nothing, Default} = Default("master"),
+            edit_link     :: Union{String, Symbol, Nothing, Default} = Default(Utilities.git_remote_head_branch("HTML(edit_link = ...)", Utilities.currentdir())),
             canonical     :: Union{String, Nothing} = nothing,
             assets        :: Vector = String[],
             analytics     :: String = "",