kopt: add fallbacks for cases where kctx is not in cache

There were a handful of cases where if there was no cached kctx there
was no fallback and several KoptInterface methods would return nil,
causing issues in various parts of KOReader (this happened with the
migration to selected_text everywhere but it's unclear how that change
caused this regression).

In any case, from a correctness perspective it makes sense to have the
corresponding fallback paths to create a new kctx if we couldn't find a
cached one.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

frontend/document/koptinterface.lua

@@ -538,19 +538,22 @@ function KoptInterface:getReflowedTextBoxes(doc, pageno)
     local hash = table.concat(hash_list, "|")
     local cached = DocCache:check(hash)
     if not cached then
+        local kc
         local kctx_hash = hash:gsub("^rfpgboxes|", "kctx|")
         cached = DocCache:check(kctx_hash)
-        if cached then
+        if not cached then
-            local kc = self:waitForContext(cached.kctx)
+            kc = self:getCachedContext(doc, pageno)
+        else
+            kc = self:waitForContext(cached.kctx)
+        end
         --kc:setDebug()
         local fullwidth, fullheight = kc:getPageDim()
         local boxes, nr_word = kc:getReflowedWordBoxes("dst", 0, 0, fullwidth, fullheight)
         if not boxes then
-                return nil
+            return
         end
         DocCache:insert(hash, CacheItem:new{ rfpgboxes = boxes, size = 192 * nr_word }) -- estimation
         return boxes
-        end
     else
         return cached.rfpgboxes
     end
@@ -566,19 +569,27 @@ function KoptInterface:getNativeTextBoxes(doc, pageno)
     local hash = table.concat(hash_list, "|")
     local cached = DocCache:check(hash)
     if not cached then
+        local kc
         local kctx_hash = hash:gsub("^nativepgboxes|", "kctx|")
         cached = DocCache:check(kctx_hash)
-        if cached then
+        if not cached then
-            local kc = self:waitForContext(cached.kctx)
+            kc = self:createContext(doc, pageno)
+            DocCache:insert(kctx_hash, ContextCacheItem:new{
+                persistent = true,
+                size = self.last_context_size or self.default_context_size,
+                kctx = kc,
+            })
+        else
+            kc = self:waitForContext(cached.kctx)
+        end
         --kc:setDebug()
         local fullwidth, fullheight = kc:getPageDim()
         local boxes, nr_word = kc:getNativeWordBoxes("dst", 0, 0, fullwidth, fullheight)
         if not boxes then
-                return nil
+            return
         end
         DocCache:insert(hash, CacheItem:new{ nativepgboxes = boxes, size = 192 * nr_word }) -- estimation
         return boxes
-        end
     else
         return cached.nativepgboxes
     end
@@ -596,21 +607,24 @@ function KoptInterface:getReflowedTextBoxesFromScratch(doc, pageno)
     local hash = table.concat(hash_list, "|")
     local cached = DocCache:check(hash)
     if not cached then
+        local reflowed_kc
         local kctx_hash = hash:gsub("^scratchrfpgboxes|", "kctx|")
         cached = DocCache:check(kctx_hash)
-        if cached then
+        if not cached then
-            local reflowed_kc = self:waitForContext(cached.kctx)
+            reflowed_kc = self:getCachedContext(doc, pageno)
+        else
+            reflowed_kc = self:waitForContext(cached.kctx)
+        end
         local fullwidth, fullheight = reflowed_kc:getPageDim()
         local kc = self:createContext(doc, pageno)
         kc:copyDestBMP(reflowed_kc)
         local boxes, nr_word = kc:getNativeWordBoxes("dst", 0, 0, fullwidth, fullheight)
         kc:free()
         if not boxes then
-                return nil
+            return
         end
         DocCache:insert(hash, CacheItem:new{ scratchrfpgboxes = boxes, size = 192 * nr_word }) -- estimation
         return boxes
-        end
     else
         return cached.scratchrfpgboxes
     end
@@ -723,17 +737,20 @@ function KoptInterface:getReflewOCRWord(doc, pageno, rect)
     local hash = table.concat(hash_list, "|")
     local cached = DocCache:check(hash)
     if not cached then
+        local kc
         local kctx_hash = hash:gsub("^rfocrword|", "kctx|")
         cached = DocCache:check(kctx_hash)
-        if cached then
+        if not cached then
-            local kc = self:waitForContext(cached.kctx)
+            kc = self:getCachedContext(doc, pageno)
+        else
+            kc = self:waitForContext(cached.kctx)
+        end
         local _, word = pcall(
             kc.getTOCRWord, kc, "dst",
             rect.x, rect.y, rect.w, rect.h,
             self.tessocr_data, self.ocr_lang, self.ocr_type, 0, 1)
         DocCache:insert(hash, CacheItem:new{ rfocrword = word, size = #word + 64 }) -- estimation
         return word
-        end
     else
         return cached.rfocrword
     end