Fix startup when history file is bad

stdlib/REPL/src/REPL.jl

@@ -1183,7 +1183,7 @@ find_hist_file() = get(ENV, "JULIA_HISTORY",
                        !isempty(DEPOT_PATH) ? joinpath(DEPOT_PATH[1], "logs", "repl_history.jl") :
                        error("DEPOT_PATH is empty and ENV[\"JULIA_HISTORY\"] not set."))
 
-backend(r::AbstractREPL) = hasproperty(r, :backendref) ? r.backendref : nothing
+backend(r::AbstractREPL) = hasproperty(r, :backendref) && isdefined(r, :backendref) ? r.backendref : nothing
 
 
 function eval_on_backend(ast, backend::REPLBackendRef)

stdlib/REPL/test/bad_history_startup.jl

@@ -0,0 +1,71 @@
+# This file is a part of Julia. License is MIT: https://julialang.org/license
+
+# Test that interactive mode starts up without error when history file is bad
+
+using Test
+
+const BASE_TEST_PATH = joinpath(Sys.BINDIR, "..", "share", "julia", "test")
+isdefined(Main, :FakePTYs) || @eval Main include(joinpath($(BASE_TEST_PATH), "testhelpers", "FakePTYs.jl"))
+import .Main.FakePTYs: with_fake_pty
+
+@testset "Bad history file startup" begin
+    mktempdir() do tmpdir
+        # Create a bad history file
+        hist_file = joinpath(tmpdir, "repl_history.jl")
+        write(hist_file, "{ invalid json content\nmore bad content\n")
+
+        julia_exe = Base.julia_cmd()[1]
+
+        # Test interactive Julia startup with bad history file
+        with_fake_pty() do pts, ptm
+            # Set up environment with our bad history file
+            nENV = copy(ENV)
+            nENV["JULIA_HISTORY"] = hist_file
+
+            # Start Julia in interactive mode
+            p = run(detach(setenv(`$julia_exe --startup-file=no --color=no -q`, nENV)), pts, pts, pts, wait=false)
+            Base.close_stdio(pts)
+
+            # Read output until we get the prompt, which indicates successful startup
+            output = readuntil(ptm, "julia> ", keep=true)
+
+            # Test conditions:
+            # 1. We should see the invalid history file error
+            has_history_error = occursin("Invalid history file", output) ||
+                              occursin("Invalid character", output)
+            @test has_history_error
+
+            # 2. We should NOT see UndefRefError (the bug being fixed)
+            has_undef_error = occursin("UndefRefError", output)
+            @test !has_undef_error
+
+            # 3. We should see the "Disabling history file" message if the fix works
+            has_disable_message = occursin("Disabling history file for this session", output)
+            @test has_disable_message
+
+            # Send exit command to clean shutdown
+            if isopen(ptm)
+                write(ptm, "exit()\n")
+            else
+                @warn "PTY master is already closed before sending exit command"
+            end
+
+            # Read any remaining output until the process exits
+            try
+                read(ptm, String)
+            catch ex
+                # Handle platform-specific EOF behavior
+                if ex isa Base.IOError && ex.code == Base.UV_EIO
+                    # This is expected on some platforms (e.g., Linux)
+                else
+                    rethrow()
+                end
+            end
+
+            # Wait for process to finish
+            wait(p)
+
+            @test p.exitcode == 0
+        end
+    end
+end

stdlib/REPL/test/runtests.jl

@@ -22,6 +22,9 @@ end
 module TerminalMenusTest
     include("TerminalMenus/runtests.jl")
 end
+module BadHistoryStartupTest
+    include("bad_history_startup.jl")
+end
 
 # Restore the original environment
 for k in keys(ENV)