This patch adds a depsafeunmerge feature that makes portage error out if we try
to unmerge a package that is currently needed by other installed packages.

--- emerge.orig	2004-11-29 09:41:28.000000000 +0100
+++ emerge	2004-12-04 01:13:56.599578783 +0100
@@ -16,7 +16,7 @@
 import portage_util
 import portage_locks
 import portage_exception
-
+import portage_dep
 
 spinner_msgs = ["Gentoo Rocks ("+os.uname()[0]+")",
                 "Thank you for using Gentoo. :)",
@@ -2041,6 +2041,22 @@
 	
 	pkgmap={}
 	numselected=0
+	vardeps={}
+	if "depsafeunmerge" in portage.features:
+		for x in portage.db["/"]["vartree"].dbapi.cpv_all():
+			vals=portage.db["/"]["vartree"].dbapi.aux_get(x, ["USE", "RDEPEND", "PDEPEND", "CDEPEND"])
+			use=vals[0].split()
+			for deps in vals[1:]:
+				deps=portage_dep.paren_reduce(deps)
+				deps=portage_dep.use_reduce(deps, use)
+				deps=portage.flatten(deps)
+				for y in deps:
+					if y[0]=="!" or y=="||":
+						continue
+					for z in portage.db["/"]["vartree"].dbapi.match(y):
+						if not vardeps.has_key(z):
+							vardeps[z]=[]
+						vardeps[z].append(x)
 	for x in candidate_catpkgs:
 		#cycle through all our candidate deps and determine what will and will not get unmerged
 		try:
@@ -2102,6 +2118,18 @@
 					pkgmap[mykey]["selected"].append(slotmap[myslot][ckey])
 					numselected=numselected+1
 				#ok, now the last-merged package is protected, and the rest are selected
+	if "depsafeunmerge" in portage.features:
+		breakloop=False
+		while not breakloop:
+			breakloop=True
+			for x in pkgmap.keys():
+				for y in pkgmap[x]["selected"]:
+					if vardeps.has_key(y):
+						for z in vardeps[y]:
+							if pkgmap.has_key(portage.dep_getkey(z)):
+								if z in pkgmap[portage.dep_getkey(z)]["selected"]:
+									breakloop=False
+									vardeps[y].remove(z)
 	if global_unmerge and not numselected:
 		print "\n>>> No outdated packages were found on your system.\n"
 		return 0
@@ -2110,6 +2138,7 @@
 		print "\n>>>",unmerge_action+": No packages selected for removal.\n"
 		return 0
 
+	unsafe=False
 	for x in pkgmap.keys():
 		for y in localtree.dep_match(x):
 			if y not in pkgmap[x]["omitted"] and \
@@ -2136,6 +2165,12 @@
 			else:
 					print "none",
 			print
+		if "depsafeunmerge" in portage.features and not pkgmap[x]["protected"]:
+			for y in pkgmap[x]["selected"]:
+				if vardeps.has_key(y) and len(vardeps[y]) > 0:
+					print red("\nUnmerging '%s' would potentially break the dependencies of the following packages:" % y)
+					print red(", ".join(vardeps[y]))
+					unsafe=True
 	
 	print "\n>>>",red("'Selected'"),"packages are slated for removal."
 	print ">>>",green("'Protected'"),"and",green("'omitted'"),"packages will not be removed.\n"
@@ -2143,6 +2178,11 @@
 	if "--pretend" in myopts:
 		#we're done... return
 		return 0
+	if unsafe:
+		myopts+=["--pretend"]
+		print "The packages you want me to unmerge would potentially break dependencies. Quitting."
+		print
+		return 0
 	if "--ask" in myopts:
 		if userquery("Do you want me to unmerge these packages?")=="No":
 			# enter pretend mode for correct formatting of results