Fix section lookup after a boundary flush

src/fstapi.c

@@ -6136,7 +6136,7 @@ char *fstReaderGetValueFromHandleAtTime(fstReaderContext *xc,
                                         fstHandle facidx,
                                         char *buf)
 {
-    fst_off_t blkpos = 0, prev_blkpos;
+    fst_off_t blkpos = 0;
     uint64_t beg_tim, end_tim, beg_tim2, end_tim2;
     int sectype;
 #ifdef FST_DEBUG
@@ -6180,7 +6180,7 @@ char *fstReaderGetValueFromHandleAtTime(fstReaderContext *xc,
     xc->rvat_chain_pos_valid = 0;
 
     for (;;) {
-        fstReaderFseeko(xc, xc->f, (prev_blkpos = blkpos), SEEK_SET);
+        fstReaderFseeko(xc, xc->f, blkpos, SEEK_SET);
 
         sectype = fgetc(xc->f);
         seclen = fstReaderUint64(xc->f);
@@ -6202,23 +6202,31 @@ char *fstReaderGetValueFromHandleAtTime(fstReaderContext *xc,
         if ((beg_tim <= tim) && (tim <= end_tim)) {
             if ((tim == end_tim) && (tim != xc->end_time)) {
                 fst_off_t cached_pos = ftello(xc->f);
-                fstReaderFseeko(xc, xc->f, blkpos, SEEK_SET);
+                fst_off_t next_blkpos = blkpos + seclen;
+                int sectype2;
+                uint64_t seclen2;
+
+                fstReaderFseeko(xc, xc->f, next_blkpos, SEEK_SET);
 
-                sectype = fgetc(xc->f);
-                seclen = fstReaderUint64(xc->f);
+                sectype2 = fgetc(xc->f);
+                seclen2 = fstReaderUint64(xc->f);
 
                 beg_tim2 = fstReaderUint64(xc->f);
                 end_tim2 = fstReaderUint64(xc->f);
 
-                if (((sectype != FST_BL_VCDATA) && (sectype != FST_BL_VCDATA_DYN_ALIAS) &&
-                     (sectype != FST_BL_VCDATA_DYN_ALIAS2)) ||
-                    (!seclen) || (beg_tim2 != tim)) {
-                    blkpos = prev_blkpos;
-                    break;
+                if (((sectype2 != FST_BL_VCDATA) && (sectype2 != FST_BL_VCDATA_DYN_ALIAS) &&
+                     (sectype2 != FST_BL_VCDATA_DYN_ALIAS2)) ||
+                    (!seclen2) || (beg_tim2 != tim)) {
+                    /* keep this section, and the read position within it */
+                    fstReaderFseeko(xc, xc->f, cached_pos, SEEK_SET);
+                } else {
+                    /* next section have the time, use it instead */
+                    sectype = sectype2;
+                    seclen = seclen2;
+                    beg_tim = beg_tim2;
+                    end_tim = end_tim2;
+                    blkpos = next_blkpos + 1;
                 }
-                beg_tim = beg_tim2;
-                end_tim = end_tim2;
-                fstReaderFseeko(xc, xc->f, cached_pos, SEEK_SET);
             }
             break;
         }

tests/meson.build

@@ -4,6 +4,7 @@ libfst_tests = [
     'empty_file',
     'mask_no_variables',
     'packed_extent_wraps',
+    'rvat_section_boundary',
     'write_and_read',
 ]
 

tests/rvat_section_boundary.c

@@ -0,0 +1,54 @@
+#include <fstapi.h>
+#include <glib.h>
+
+// Random access reads at the end time of a value change section: one where
+// the next section begins at that same time, and one at the end of the last
+// value change section, in a file that ends later.
+
+int main(int argc, char **argv)
+{
+    (void)argc;
+    (void)argv;
+
+    fstWriterContext *writer = fstWriterCreate("rvat_section_boundary.fst", 1);
+    g_assert_true(writer != NULL);
+
+    fstHandle var = fstWriterCreateVar(writer, FST_VT_VCD_WIRE, FST_VD_IMPLICIT, 1, "var", 0);
+
+    fstWriterEmitTimeChange(writer, 0);
+    fstWriterEmitValueChange(writer, var, "0");
+    fstWriterEmitTimeChange(writer, 1);
+    fstWriterEmitValueChange(writer, var, "1");
+    fstWriterEmitTimeChange(writer, 2);
+    fstWriterEmitValueChange(writer, var, "0");
+
+    // The section is flushed at the next time change: it ends at 2, and the
+    // next one begins at 2.
+    fstWriterFlushContext(writer);
+
+    fstWriterEmitTimeChange(writer, 3);
+    fstWriterEmitValueChange(writer, var, "1");
+    fstWriterEmitTimeChange(writer, 4);
+    fstWriterEmitValueChange(writer, var, "0");
+
+    // Same again, ending the second section at 4; the third one holds no value
+    // changes, so it is not written, but the file still ends at 5.
+    fstWriterFlushContext(writer);
+
+    fstWriterEmitTimeChange(writer, 5);
+
+    fstWriterClose(writer);
+
+    fstReaderContext *reader = fstReaderOpen("rvat_section_boundary.fst");
+    g_assert_true(reader != NULL);
+    g_assert_cmpint(fstReaderGetEndTime(reader), ==, 5);
+
+    char buf[64];
+
+    g_assert_cmpstr(fstReaderGetValueFromHandleAtTime(reader, 2, var, buf), ==, "0");
+    g_assert_cmpstr(fstReaderGetValueFromHandleAtTime(reader, 4, var, buf), ==, "0");
+
+    fstReaderClose(reader);
+
+    return 0;
+}