nix-env -i: Add a flag ‘--remove-all’ / ‘-r’

This is equivalent to running ‘nix-env -e '*'’ first, except that it
happens in a single transaction.  Thus, ‘nix-env -i pkgs...’ replaces
the profile with the specified set of packages.

The main motivation is to support declarative package management
(similar to environment.systemPackages in NixOS).  That is, if you
have a specification ‘profile.nix’ like this:

  with import <nixpkgs> {};
  [ thunderbird
    geeqie
    ...
  ]

then after any change to ‘profile.nix’, you can run:

  $ nix-env -f profile.nix -ir

to update the profile to match the specification.  (Without the ‘-r’
flag, if you remove a package from ‘profile.nix’, it won't be removed
from the actual profile.)

Suggested by @zefhemel.

doc/manual/nix-env.xml

@@ -209,6 +209,10 @@ also <xref linkend="sec-common-options" />.</phrase></para>
     <arg choice='plain'><option>--preserve-installed</option></arg>
     <arg choice='plain'><option>-P</option></arg>
   </group>
+  <group choice='opt'>
+    <arg choice='plain'><option>--remove-all</option></arg>
+    <arg choice='plain'><option>-r</option></arg>
+  </group>
   <arg choice='plain' rep='repeat'><replaceable>args</replaceable></arg>
 </cmdsynopsis>
 
@@ -318,6 +322,16 @@ number of possible ways:
 
   </varlistentry>
 
+  <varlistentry><term><option>--remove-all</option></term>
+    <term><option>-r</option></term>
+
+    <listitem><para>Remove all previously installed packages first.
+    This is equivalent to running <literal>nix-env -e '*'</literal>
+    first, except that everything happens in a single
+    transaction.</para></listitem>
+
+  </varlistentry>
+
 </variablelist>
 
 </refsection>

src/nix-env/nix-env.cc

@@ -54,6 +54,7 @@ struct Globals
     EvalState state;
     bool dryRun;
     bool preserveInstalled;
+    bool removeAll;
     string forceName;
     bool prebuiltOnly;
 };
@@ -489,29 +490,33 @@ static void installDerivations(Globals & globals,
         newNames.insert(DrvName(i->name).name);
     }
 
-    /* Add in the already installed derivations, unless they have the
-       same name as a to-be-installed element. */
 
     while (true) {
         string lockToken = optimisticLockProfile(profile);
 
+        DrvInfos allElems(newElems);
+
+        /* Add in the already installed derivations, unless they have
+           the same name as a to-be-installed element. */
+        if (!globals.removeAll) {
+            printMsg(lvlError, "FOO");
+            
             DrvInfos installedElems = queryInstalled(globals.state, profile);
 
-        DrvInfos allElems(newElems);
             foreach (DrvInfos::iterator, i, installedElems) {
                 DrvName drvName(i->name);
                 MetaInfo meta = i->queryMetaInfo(globals.state);
                 if (!globals.preserveInstalled &&
                     newNames.find(drvName.name) != newNames.end() &&
                     !keep(meta))
-                printMsg(lvlInfo,
-                    format("replacing old `%1%'") % i->name);
+                    printMsg(lvlInfo, format("replacing old `%1%'") % i->name);
                 else
                     allElems.push_back(*i);
             }
 
             foreach (DrvInfos::iterator, i, newElems)
                 printMsg(lvlInfo, format("installing `%1%'") % i->name);
+        }
 
         printMissing(globals.state, newElems);
 
@@ -531,6 +536,8 @@ static void opInstall(Globals & globals,
         if (parseInstallSourceOptions(globals, i, opFlags, arg)) ;
         else if (arg == "--preserve-installed" || arg == "-P")
             globals.preserveInstalled = true;
+        else if (arg == "--remove-all" || arg == "-r")
+            globals.removeAll = true;
         else throw UsageError(format("unknown flag `%1%'") % arg);
     }
 
@@ -1298,6 +1305,7 @@ void run(Strings args)
 
     globals.dryRun = false;
     globals.preserveInstalled = false;
+    globals.removeAll = false;
     globals.prebuiltOnly = false;
 
     for (Strings::iterator i = args.begin(); i != args.end(); ) {