repl: log errors writing to history file

These errors are now logged and explicitly ignored, rather than
implicitly ignored.

Change-Id: Ia26015466a17f2b11952df5317a4d150d79dc184

src/libcmd/repl-interacter.cc

@@ -1,6 +1,8 @@
 #include <cstdio>
 #include <iostream>
 #include <string>
+#include <string_view>
+#include <cerrno>
 
 #ifdef READLINE
 #include <readline/history.h>
@@ -176,15 +178,27 @@ bool ReadlineLikeInteracter::getLine(std::string & input, ReplPromptType promptT
     if (!s)
         return false;
 
-    write_history(historyFile.c_str());
+    this->writeHistory();
     input += s;
     input += '\n';
     return true;
 }
 
+void ReadlineLikeInteracter::writeHistory()
+{
+    if (write_history(historyFile.c_str()) != 0) {
+        // write_history returns the result of fclose() (which also flushes).
+        // We should explicitly ignore these errors, but log them so the user
+        // isn't confused why their history is getting eaten.
+        int const fcloseErr = errno;
+        std::string_view const errMsg(std::strerror(fcloseErr));
+        warn("ignoring error writing repl history to %s: %s", this->historyFile, errMsg);
+    }
+}
+
 ReadlineLikeInteracter::~ReadlineLikeInteracter()
 {
-    write_history(historyFile.c_str());
+    this->writeHistory();
 }
 
 AutomationInteracter::Guard AutomationInteracter::init(detail::ReplCompleterMixin *)

src/libcmd/repl-interacter.hh

@@ -42,6 +42,11 @@ public:
     }
     virtual Guard init(detail::ReplCompleterMixin * repl) override;
     virtual bool getLine(std::string & input, ReplPromptType promptType) override;
+    /** Writes the current history to the history file.
+     *
+     * This function logs but ignores errors from readline's write_history().
+     */
+    virtual void writeHistory();
     virtual ~ReadlineLikeInteracter() override;
 };