fetchurl: add user agent

It would be nice to be able to track Nix requests. It's not trustworthy,
but can be helpful for stats and routing in HTTP logs.

Since `fetchurl` is used so widely, we should "magically" get a UA on
`fetchzip`, `fetchFromGitHub`, and other related fetchers.

Since `fetchurl` is only used for fixed-output derivations, this should
cause no mass rebuild.

User-Agent example: curl/7.57.0 Nixpkgs/18.03

pkgs/build-support/fetchurl/builder.sh

@@ -2,20 +2,23 @@ source $stdenv/setup
 
 source $mirrorsFile
 
+curlVersion=$(curl -V | head -1 | cut -d' ' -f2)
 
 # Curl flags to handle redirects, not use EPSV, handle cookies for
 # servers to need them during redirects, and work on SSL without a
 # certificate (this isn't a security problem because we check the
 # cryptographic hash of the output anyway).
-curl="curl \
- --location --max-redirs 20 \
- --retry 3 \
- --disable-epsv \
- --cookie-jar cookies \
- --insecure \
- $curlOpts \
- $NIX_CURL_FLAGS"
-
+curl=(
+    curl
+    --location
+    --max-redirs 20
+    --disable-epsv
+    --cookie-jar cookies
+    --insecure
+    --user-agent "curl/$curlVersion Nixpkgs/$nixpkgsVersion"
+    $curlOpts
+    $NIX_CURL_FLAGS
+)
 
 downloadedFile="$out"
 if [ -n "$downloadToTemp" ]; then downloadedFile="$TMPDIR/file"; fi
@@ -32,7 +35,7 @@ tryDownload() {
     # if we get error code 18, resume partial download
     while [ $curlexit -eq 18 ]; do
        # keep this inside an if statement, since on failure it doesn't abort the script
-       if $curl -C - --fail "$url" --output "$downloadedFile"; then
+       if "${curl[@]}" -C - --fail "$url" --output "$downloadedFile"; then
           success=1
           break
        else
@@ -61,7 +64,7 @@ tryHashedMirrors() {
 
     for mirror in $hashedMirrors; do
         url="$mirror/$outputHashAlgo/$outputHash"
-        if $curl --retry 0 --connect-timeout "${NIX_CONNECT_TIMEOUT:-15}" \
+        if "${curl[@]}" --retry 0 --connect-timeout "${NIX_CONNECT_TIMEOUT:-15}" \
             --fail --silent --show-error --head "$url" \
             --write-out "%{http_code}" --output /dev/null > code 2> log; then
             tryDownload "$url"

pkgs/build-support/fetchurl/default.nix

@@ -95,7 +95,7 @@ assert sha512 != "" -> builtins.compareVersions "1.11" builtins.nixVersion <= 0;
 
 
 let
-
+  inherit (stdenv.lib) fileContents;
   hasHash = showURLs || (outputHash != "" && outputHashAlgo != "")
     || sha1 != "" || sha256 != "" || sha512 != "";
   urls_ = if urls != [] then urls else [url];
@@ -132,6 +132,8 @@ else stdenv.mkDerivation {
 
   impureEnvVars = impureEnvVars ++ netrcImpureEnvVars;
 
+  nixpkgsVersion = fileContents ../../../.version;
+
   # Doing the download on a remote machine just duplicates network
   # traffic, so don't do that.
   preferLocalBuild = true;