From 377d5eb38815d8290ab444c81c7826f46ba79844 Mon Sep 17 00:00:00 2001
From: dramforever <dramforever@live.com>
Date: Mon, 30 Jan 2023 23:56:27 +0800
Subject: [PATCH] Installable::getCursors: Cleanup

- Clarify doc comments, Installables::getCursors returns non-empty
  vector
- Use vector::at in Installable::getCursor instead of checking for empty
  vector and throwing an exception with error message.
---
 src/libcmd/installables.cc | 7 +++----
 src/libcmd/installables.hh | 8 ++++++--
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index f8632a535..24f458f1a 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -379,10 +379,9 @@ Installable::getCursors(EvalState & state)
 ref<eval_cache::AttrCursor>
 Installable::getCursor(EvalState & state)
 {
-    auto cursors = getCursors(state);
-    if (cursors.empty())
-        throw Error("cannot find flake attribute '%s'", what());
-    return cursors[0];
+    /* Although getCursors should return at least one element, in case it doesn't,
+       bound check to avoid an undefined behavior for vector[0] */
+    return getCursors(state).at(0);
 }
 
 static StorePath getDeriver(
diff --git a/src/libcmd/installables.hh b/src/libcmd/installables.hh
index d489315df..da6a3addd 100644
--- a/src/libcmd/installables.hh
+++ b/src/libcmd/installables.hh
@@ -103,9 +103,13 @@ struct Installable
         return {};
     }
 
+    /* Get a cursor to each value this Installable could refer to. However
+       if none exists, throw exception instead of returning empty vector. */
     virtual std::vector<ref<eval_cache::AttrCursor>>
     getCursors(EvalState & state);
 
+    /* Get the first and most preferred cursor this Installable could refer
+       to, or throw an exception if none exists. */
     virtual ref<eval_cache::AttrCursor>
     getCursor(EvalState & state);
 
@@ -193,8 +197,8 @@ struct InstallableFlake : InstallableValue
 
     std::pair<Value *, PosIdx> toValue(EvalState & state) override;
 
-    /* Get a cursor to every attrpath in getActualAttrPaths() that
-       exists. */
+    /* Get a cursor to every attrpath in getActualAttrPaths()
+       that exists. However if none exists, throw an exception. */
     std::vector<ref<eval_cache::AttrCursor>>
     getCursors(EvalState & state) override;