Disallow store path names that are . or .. (plus opt. -)

As discussed in the maintainer meeting on 2024-01-29.

Mainly this is to avoid a situation where the name is parsed and
treated as a file name, mostly to protect users.
.-* and ..-* are also considered invalid because they might strip
on that separator to remove versions. Doesn't really work, but that's
what we decided, and I won't argue with it, because .-* probably
doesn't seem to have a real world application anyway.
We do still permit a 1-character name that's just "-", which still
poses a similar risk in such a situation. We can't start disallowing
trailing -, because a non-zero number of users will need it and we've
seen how annoying and painful such a change is.

What matters most is preventing a situation where . or .. can be
injected, and to just get this done.

(cherry picked from commit f1b4663805a9dbcb1ace64ec110092d17c9155e0)
Change-Id: I900a8509933cee662f888c3c76fa8986b0058839

src/libstore/path-regex.hh

@@ -3,6 +3,11 @@
 
 namespace nix {
 
-static constexpr std::string_view nameRegexStr = R"([0-9a-zA-Z\+\-\._\?=]+)";
+
+static constexpr std::string_view nameRegexStr =
+    // This uses a negative lookahead: (?!\.\.?(-|$))
+    //   - deny ".", "..", or those strings followed by '-'
+    //   - when it's not those, start again at the start of the input and apply the next regex, which is [0-9a-zA-Z\+\-\._\?=]+
+    R"((?!\.\.?(-|$))[0-9a-zA-Z\+\-\._\?=]+)";
 
 }

src/libstore/path.cc

@@ -12,6 +12,19 @@ static void checkName(std::string_view path, std::string_view name)
         throw BadStorePath("store path '%s' has a name longer than %d characters",
             path, StorePath::MaxPathLen);
     // See nameRegexStr for the definition
+    if (name[0] == '.') {
+        // check against "." and "..", followed by end or dash
+        if (name.size() == 1)
+            throw BadStorePath("store path '%s' has invalid name '%s'", path, name);
+        if (name[1] == '-')
+            throw BadStorePath("store path '%s' has invalid name '%s': first dash-separated component must not be '%s'", path, name, ".");
+        if (name[1] == '.') {
+            if (name.size() == 2)
+                throw BadStorePath("store path '%s' has invalid name '%s'", path, name);
+            if (name[2] == '-')
+                throw BadStorePath("store path '%s' has invalid name '%s': first dash-separated component must not be '%s'", path, name, "..");
+        }
+    }
     for (auto c : name)
         if (!((c >= '0' && c <= '9')
                 || (c >= 'a' && c <= 'z')