Add a setting for configuring the SSL certificates file

This provides a platform-independent way to configure the SSL
certificates file in the Nix daemon. Previously we provided
instructions for overriding the environment variable in launchd, but
that obviously doesn't work with systemd. Now we can just tell users
to add

  ssl-cert-file = /etc/ssl/my-certificate-bundle.crt

to their nix.conf.

doc/manual/src/installation/env-variables.md

@@ -42,14 +42,11 @@ export NIX_SSL_CERT_FILE=/etc/ssl/my-certificate-bundle.crt
 > You must not add the export and then do the install, as the Nix
 > installer will detect the presence of Nix configuration, and abort.
 
-## `NIX_SSL_CERT_FILE` with macOS and the Nix daemon
+If you use the Nix daemon, you should also add the following to
+`/etc/nix/nix.conf`:
 
-On macOS you must specify the environment variable for the Nix daemon
-service, then restart it:
-
-```console
-$ sudo launchctl setenv NIX_SSL_CERT_FILE /etc/ssl/my-certificate-bundle.crt
-$ sudo launchctl kickstart -k system/org.nixos.nix-daemon
+```
+ssl-cert-file = /etc/ssl/my-certificate-bundle.crt
 ```
 
 ## Proxy Environment Variables

misc/launchd/org.nixos.nix-daemon.plist.in

@@ -4,8 +4,6 @@
   <dict>
     <key>EnvironmentVariables</key>
     <dict>
-      <key>NIX_SSL_CERT_FILE</key>
-      <string>/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt</string>
       <key>OBJC_DISABLE_INITIALIZE_FORK_SAFETY</key>
       <string>YES</string>
     </dict>

src/libstore/filetransfer.cc

@@ -318,7 +318,7 @@ struct curlFileTransfer : public FileTransfer
 
             if (request.verifyTLS) {
                 if (settings.caFile != "")
-                    curl_easy_setopt(req, CURLOPT_CAINFO, settings.caFile.c_str());
+                    curl_easy_setopt(req, CURLOPT_CAINFO, settings.caFile.get().c_str());
             } else {
                 curl_easy_setopt(req, CURLOPT_SSL_VERIFYPEER, 0);
                 curl_easy_setopt(req, CURLOPT_SSL_VERIFYHOST, 0);

src/libstore/globals.cc

@@ -44,14 +44,9 @@ Settings::Settings()
     lockCPU = getEnv("NIX_AFFINITY_HACK") == "1";
     allowSymlinkedStore = getEnv("NIX_IGNORE_SYMLINK_STORE") == "1";
 
-    caFile = getEnv("NIX_SSL_CERT_FILE").value_or(getEnv("SSL_CERT_FILE").value_or(""));
-    if (caFile == "") {
-        for (auto & fn : {"/etc/ssl/certs/ca-certificates.crt", "/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt"})
-            if (pathExists(fn)) {
-                caFile = fn;
-                break;
-            }
-    }
+    auto sslOverride = getEnv("NIX_SSL_CERT_FILE").value_or(getEnv("SSL_CERT_FILE").value_or(""));
+    if (sslOverride != "")
+        caFile = sslOverride;
 
     /* Backwards compatibility. */
     auto s = getEnv("NIX_REMOTE_SYSTEMS");
@@ -187,6 +182,13 @@ bool Settings::isWSL1()
     return hasSuffix(utsbuf.release, "-Microsoft");
 }
 
+Path Settings::getDefaultSSLCertFile()
+{
+    for (auto & fn : {"/etc/ssl/certs/ca-certificates.crt", "/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt"})
+        if (pathExists(fn)) return fn;
+    return "";
+}
+
 const std::string nixVersion = PACKAGE_VERSION;
 
 NLOHMANN_JSON_SERIALIZE_ENUM(SandboxMode, {

src/libstore/globals.hh

@@ -64,6 +64,8 @@ class Settings : public Config {
 
     bool isWSL1();
 
+    Path getDefaultSSLCertFile();
+
 public:
 
     Settings();
@@ -826,8 +828,17 @@ public:
           > `.netrc`.
         )"};
 
-    /* Path to the SSL CA file used */
-    Path caFile;
+    Setting<Path> caFile{
+        this, getDefaultSSLCertFile(), "ssl-cert-file",
+        R"(
+          The path of a file containing CA certificates used to
+          authenticate `https://` downloads. It defaults to the first
+          of `/etc/ssl/certs/ca-certificates.crt` and
+          `/nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt`
+          that exists. It can be overriden using the
+          `NIX_SSL_CERT_FILE` and `SSL_CERT_FILE` environment variable
+          (in that order of precedence).
+        )"};
 
 #if __linux__
     Setting<bool> filterSyscalls{