nixos/test-driver: Make tesseract OCR optional.

By default this is now enabled, and it has to be explicitely enabled
using "enableOCR = true". If it is set to false, any usage of
getScreenText or waitForText will fail with an error suggesting to pass
enableOCR.

This should get rid of the rather large dependency on tesseract which
we don't need for most tests.

Note, that I'm using system("type -P") here to check whether tesseract
is in PATH. I know it's a bashism but we already have other bashisms
within the test scripts and we also run it with bash, so IMHO it's not a
problem here.

Signed-off-by: aszlig <[e-mail]>

nixos/doc/manual/development/writing-nixos-tests.xml

···
     <term><methodname>getScreenText</methodname></term>
     <listitem><para>Return a textual representation of what is currently
     visible on the machine's screen using optical character
-    recognition.</para></listitem>
+    recognition.</para>
+    <note><para>This requires passing <option>enableOCR</option> to the test
+    attribute set.</para></note></listitem>
   </varlistentry>
 
   <varlistentry>
···
     <term><methodname>waitForText</methodname></term>
     <listitem><para>Wait until the supplied regular expressions matches
     the textual contents of the screen by using optical character recognition
-    (see <methodname>getScreenText</methodname>).</para></listitem>
+    (see <methodname>getScreenText</methodname>).</para>
+    <note><para>This requires passing <option>enableOCR</option> to the test
+    attribute set.</para></note></listitem>
   </varlistentry>
 
   <varlistentry>

nixos/lib/test-driver/Machine.pm

···
 sub getScreenText {
     my ($self) = @_;
 
+    system("type -P tesseract &> /dev/null") == 0
+        or die "getScreenText used but enableOCR is false";
+
     my $text;
     $self->nest("performing optical character recognition", sub {
         my $tmpbase = Cwd::abs_path(".")."/ocr";

nixos/lib/testing.nix

···
         cp ${./test-driver/Logger.pm} $libDir/Logger.pm
 
         wrapProgram $out/bin/nixos-test-driver \
-          --prefix PATH : "${qemu_kvm}/bin:${vde2}/bin:${netpbm}/bin:${coreutils}/bin:${tesseract}/bin" \
+          --prefix PATH : "${qemu_kvm}/bin:${vde2}/bin:${netpbm}/bin:${coreutils}/bin" \
           --prefix PERL5LIB : "${with perlPackages; lib.makePerlPath [ TermReadLineGnu XMLWriter IOTty FileSlurp ]}:$out/lib/perl5/site_perl"
       '';
   };
···
 
 
   makeTest =
-    { testScript, makeCoverageReport ? false, name ? "unnamed", ... } @ t:
+    { testScript
+    , makeCoverageReport ? false
+    , enableOCR ? false
+    , name ? "unnamed"
+    , ...
+    } @ t:
 
     let
       testDriverName = "nixos-test-driver-${name}";
···
           vms="$(for i in ${toString vms}; do echo $i/bin/run-*-vm; done)"
           wrapProgram $out/bin/nixos-test-driver \
             --add-flags "$vms" \
+            ${lib.optionalString enableOCR "--prefix PATH : '${tesseract}/bin'"} \
             --run "testScript=\"\$(cat $out/test-script)\"" \
             --set testScript '"$testScript"' \
             --set VLANS '"${toString vlans}"'
           ln -s ${testDriver}/bin/nixos-test-driver $out/bin/nixos-run-vms
           wrapProgram $out/bin/nixos-run-vms \
             --add-flags "$vms" \
+            ${lib.optionalString enableOCR "--prefix PATH : '${tesseract}/bin'"} \
             --set tests '"startAll; joinAll;"' \
             --set VLANS '"${toString vlans}"' \
             ${lib.optionalString (builtins.length vms == 1) "--set USE_SERIAL 1"}

nixos/tests/installer.nix

···
   makeInstallerTest = name:
     { createPartitions, preBootCommands ? "", extraConfig ? ""
     , testChannel ? false, grubVersion ? 2, grubDevice ? "/dev/vda"
-    , grubIdentifier ? "uuid"
+    , grubIdentifier ? "uuid", enableOCR ? false
     }:
     makeTest {
       inherit iso;
       name = "installer-" + name;
       nodes = if testChannel then { inherit webserver; } else { };
+      inherit enableOCR;
       testScript = testScriptFun {
         inherit createPartitions preBootCommands testChannel grubVersion
                 grubDevice grubIdentifier extraConfig;
···
           preLVM = true;
         };
       '';
+      enableOCR = true;
       preBootCommands = ''
         $machine->start;
         $machine->waitForText(qr/Enter passphrase/);