fakebts: Make it easy to do a LU and get the current TMSI from it

Tests that require sending a CM Service Request require a TMSI. Add
a selector that helps to get the TMSI so it can be used in tests.

fakebts/OpenBSCTest.st

@@ -97,6 +97,7 @@ Object subclass: OpenBSCTest [
     | bts testFailed |
     <category: 'OpenBSC-Test'>
     <comment: 'I help in dealing with setup and teardown of a test'>
+    <import: OsmoGSM>
 
     OpenBSCTest class >> initWith: aBTS [
         <category: 'creation'>
@@ -176,5 +177,56 @@ Object subclass: OpenBSCTest [
         <category: 'verifying'>
         ^ testFailed
     ]
+
+    allocateTmsi: imsi [
+        | tmsi lchan lu msg |
+        "Do a LU and get the TMSI."
+        "2. Get a LCHAN"
+        lchan := self requireAnyChannel.
+
+        "3. Send the LU request"
+        lu := GSM48LURequest new.
+        lu lai
+            mcc: 1;
+            mnc: 1;
+            lac: 1.
+        lu mi imsi: imsi.
+        lchan sendGSM: lu toMessage.
+
+        "Now deal with what the NITB wants"
+        "4.1 Send the IMEI..."
+        msg := GSM48MSG decode: lchan nextSapi0Msg readStream.
+        (msg isKindOf: GSM48IdentityReq)
+            ifFalse: [^self error: 'Wanted identity request'].
+        (msg idType isIMEI)
+            ifFalse: [^self error: 'Wanted IMEI reqest'].
+        msg := GSM48IdentityResponse new.
+        msg mi imei: '6666666666666666'.
+        lchan sendGSM: msg toMessage.
+
+        "4.2 LU Accept"
+        msg := GSM48MSG decode: lchan nextSapi0Msg readStream.
+        (msg isKindOf: GSM48LUAccept)
+            ifFalse: [^self error: 'LU failed'].
+        tmsi := msg mi tmsi.
+        msg := GSM48TMSIReallocationComplete new.
+        lchan sendGSM: msg toMessage.
+
+        "4.3 MM Information for the time. ignore it"
+        msg := GSM48MSG decode: lchan nextSapi0Msg readStream.
+        (msg isKindOf: GSM48MMInformation)
+            ifFalse: [^self error: 'MM Information'].
+
+        "4.4 release.. if we now don't close the LCHAN it will
+        remain open for a bit. OpenBSC should and will start the
+        approriate timer soon(tm)"
+        msg := GSM48MSG decode: lchan nextSapi0Msg readStream.
+        (msg isKindOf: GSM48RRChannelRelease)
+            ifFalse: [^self error: 'RR Channel Release'].
+
+        "4.5.. be nice... for now and send a disconnect."
+        lchan releaseAllSapis.
+        ^ tmsi.
+    ]
 ]