libstore: ignore broken symlinks in ssl-cert-file default

Also tweak `pathAccessible` to ignore other relevant errors too. It was
documented as ignoring permission errors but it was only ignoring
`EPERM`, which comes from the darwin sandbox, and not ignoring `EACCESS`
which is the real permission error. I figured it also makes sense to
ignore `ELOOP`.

Fixes: #560
Change-Id: Ibb849b68d07386eb80afb52b57f7d12b3a48a202

doc/manual/rl-next/ssl-cert-symlink.md

@@ -0,0 +1,12 @@
+---
+synopsis: Ignore broken `/etc/ssl/certs/ca-certificates.crt` symlink
+issues: [fj#560]
+cls: [2144]
+category: Fixes
+credits: lilyball
+---
+
+[`ssl-cert-file`](@docroot@/command-ref/conf-file.md#conf-ssl-cert-file) now checks its default
+value for a broken symlink before using it. This fixes a problem on macOS where uninstalling
+nix-darwin may leave behind a broken symlink at `/etc/ssl/certs/ca-certificates.crt` that was
+stopping Lix from using the cert at `/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt`.

src/libstore/globals.cc

@@ -263,7 +263,7 @@ bool Settings::isWSL1()
 Path Settings::getDefaultSSLCertFile()
 {
     for (auto & fn : {"/etc/ssl/certs/ca-certificates.crt", "/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt"})
-        if (pathAccessible(fn)) return fn;
+        if (pathAccessible(fn, true)) return fn;
     return "";
 }
 

src/libutil/file-system.cc

@@ -214,6 +214,19 @@ struct stat lstat(const Path & path)
     return st;
 }
 
+std::optional<struct stat> maybeStat(const Path & path)
+{
+    std::optional<struct stat> st{std::in_place};
+    if (stat(path.c_str(), &*st))
+    {
+        if (errno == ENOENT || errno == ENOTDIR)
+            st.reset();
+        else
+            throw SysError("getting status of '%s'", path);
+    }
+    return st;
+}
+
 std::optional<struct stat> maybeLstat(const Path & path)
 {
     std::optional<struct stat> st{std::in_place};
@@ -232,16 +245,25 @@ bool pathExists(const Path & path)
     return maybeLstat(path).has_value();
 }
 
-bool pathAccessible(const Path & path)
+bool pathAccessible(const Path & path, bool resolveSymlinks)
 {
     try {
-        return pathExists(path);
+        return resolveSymlinks ? maybeStat(path).has_value() : pathExists(path);
     } catch (SysError & e) {
-        // swallow EPERM
-        if (e.errNo == EPERM) return false;
+        switch (e.errNo) {
+        case EPERM:
+            // operation not permitted error, can occur in darwin sandbox
+        case EACCES:
+            // permission error
+        case ELOOP:
+            // path component is a looping symlink
+            // this seems like a reasonable condition to handle as well
+            return false;
+        default:
             throw;
         }
     }
+}
 
 
 Path readLink(const Path & path)

src/libutil/file-system.hh

@@ -123,6 +123,12 @@ bool isDirOrInDir(std::string_view path, std::string_view dir);
 struct stat stat(const Path & path);
 struct stat lstat(const Path & path);
 
+/**
+ * `stat` the given path if it exists.
+ * @return std::nullopt if the path doesn't exist, or an optional containing the result of `stat` otherwise
+ */
+std::optional<struct stat> maybeStat(const Path & path);
+
 /**
  * `lstat` the given path if it exists.
  * @return std::nullopt if the path doesn't exist, or an optional containing the result of `lstat` otherwise
@@ -137,10 +143,11 @@ bool pathExists(const Path & path);
 /**
  * A version of pathExists that returns false on a permission error.
  * Useful for inferring default paths across directories that might not
- * be readable.
+ * be readable. Optionally resolves symlinks to determine if the real
+ * path exists.
  * @return true iff the given path can be accessed and exists
  */
-bool pathAccessible(const Path & path);
+bool pathAccessible(const Path & path, bool resolveSymlinks = false);
 
 /**
  * Read the contents (target) of a symbolic link.  The result is not