fix: extsort underflow in CSV mode

resources/test/issue2391-test_ids.csv

@@ -0,0 +1,17 @@
+pnm,tc_id,pc_id
+405,139280,9730000630075
+405,139281,9730000630075
+131,139282862,9730065908379
+138,139282863,9730065908379
+138,139282864,9730065908379
+405,139282865,9730065908379
+138,139282866,9730065908379
+138,139282867,9730065908379
+138,139282868,9730065908379
+138,139282869,9730065908379
+138,139282870,9730065908379
+138,139282871,9730065908379
+252,139282,9730000630075
+241,139283,9730000630075
+272,139284,9730000630075
+273,139285,9730000630075

src/cmd/extsort.rs

@@ -237,7 +237,7 @@ fn sort_csv(
             return fail!("Failed to retrieve position: invalid integer");
         };
 
-        idxfile.seek(position - position_delta)?;
+        idxfile.seek(position.saturating_sub(position_delta))?;
         idxfile.read_byte_record(&mut record_wrk)?;
         sorted_csv_wtr.write_byte_record(&record_wrk)?;
     }

tests/test_extsort.rs

@@ -53,3 +53,68 @@ fn extsort_csvmode() {
 
     assert_eq!(dos2unix(&sorted_output), dos2unix(&expected_csv));
 }
+
+#[test]
+fn extsort_issue_2391() {
+    let wrk = Workdir::new("extsort_issue_2391").flexible(true);
+    wrk.clear_contents().unwrap();
+
+    let unsorted_csv = wrk.load_test_resource("issue2391-test_ids.csv");
+    wrk.create_from_string("issue2391-test_ids.csv", &unsorted_csv);
+    // create index
+    let mut cmd_wrk = wrk.command("index");
+    cmd_wrk.arg("issue2391-test_ids.csv");
+
+    wrk.assert_success(&mut cmd_wrk);
+
+    // as git mangles line endings, we need to convert manually to CRLF as per issue 2391
+    // see https://github.com/dathere/qsv/issues/2391
+    // convert LF to CRLF in test file to ensure consistent line endings
+    #[cfg(target_os = "windows")]
+    {
+        let mut cmd = wrk.command("cmd");
+        cmd.args([
+            "/C",
+            "type issue2391-test_ids.csv > issue2391-test_ids.tmp.csv && move /Y \
+             issue2391-test_ids.tmp.csv issue2391-test_ids.csv",
+        ]);
+        wrk.output(&mut cmd);
+    }
+    #[cfg(not(target_os = "windows"))]
+    {
+        let mut cmd = wrk.command("sh");
+        cmd.args([
+            "-c",
+            "sed 's/$/\r/' issue2391-test_ids.csv > issue2391-test_ids.tmp.csv && mv \
+             issue2391-test_ids.tmp.csv issue2391-test_ids.csv",
+        ]);
+        wrk.output(&mut cmd);
+    }
+
+    let mut cmd = wrk.command("extsort");
+    cmd.arg("issue2391-test_ids.csv")
+        .args(["--select", "tc_id,pnm,pc_id"]);
+
+    wrk.assert_success(&mut cmd);
+    let got: Vec<Vec<String>> = wrk.read_stdout(&mut cmd);
+    let expected = vec![
+        svec!["pnm", "tc_id", "pc_id"],
+        svec!["405", "139280", "9730000630075"],
+        svec!["405", "139281", "9730000630075"],
+        svec!["252", "139282", "9730000630075"],
+        svec!["131", "139282862", "9730065908379"],
+        svec!["138", "139282863", "9730065908379"],
+        svec!["138", "139282864", "9730065908379"],
+        svec!["405", "139282865", "9730065908379"],
+        svec!["138", "139282866", "9730065908379"],
+        svec!["138", "139282867", "9730065908379"],
+        svec!["138", "139282868", "9730065908379"],
+        svec!["138", "139282869", "9730065908379"],
+        svec!["138", "139282870", "9730065908379"],
+        svec!["138", "139282871", "9730065908379"],
+        svec!["241", "139283", "9730000630075"],
+        svec!["272", "139284", "9730000630075"],
+        svec!["273", "139285", "9730000630075"],
+    ];
+    assert_eq!(got, expected);
+}